This works. It could be placed in someone's Lab, if they are willing to accept -1 Safety, and +1 Warping.
That's a net negative. Why would anyone want it there? I assumed that it was kept safe (in the vis vault?) and taken out once a year to be burned.
How about some Imaginem:
Narcissus's Pool
In a forest glen not far from Thessaloniki is a small pool fed by a stream. If a person walks around the pool three times while never losing sight of their reflection, they are transported to an associated regio that looks much like the forest glen except that the pool of water has a faded image permanently in it. The faded image is that of an ancient Greek man of unsurpassed beauty. In fact, his appearance is so striking that it's hard to look away. The magus who found the pool theorized that only the fact that the image is faded allows a person to turn away at all. However, when the sun is brightest, at noon on the day of the summer solstice, the image brightens and regains some of its former beauty, captivating any who see it. Thankfully the moment passes and the image fades once more, allowing the gazer to look away once more. But if, during that moment of clarity some water is drawn from the pool where the image is formed, that water will contain 3 pawns of Imaginem vis. One would be wise to have everything at hand before the sun reaches its zenith however. For once the image brightens, it's impossible to look away to gather what's needed.
Provides: 3 Im vis/year in the summer.
Another magic item:
Amulet of Purifcation:
This is a device to rid the world of demons. It's a long stretch of brass chain, with an amulet, bearing the sigils of Angels in the middle. It provides a Perdo Vim effect, destroying 11 Might of an Infernal creature. When the chain is wrapped tightly around an entity, the effect is cast every round.
Pen +60, Unlimited uses/day PeVi 45 (Base 1, +1 Touch, +3 Environmental Trigger, +30 Penetration, +10 Unlimited uses)
Costs 18 BP.
No hidden demons in your covenant!
BTW, this leaves us with 10 BP of magic items left to choose . (We only had 70 BP of Magic items.) that’s one level 25 item.
+2 to the on the appropriate Art. page 118, Covenants.
I like it, especially that it's perilous.
How about a Signaficato? (Covenants, page 101)
The Giant's Goblet
On the coast of the Island, in the Regio, water flows into a large natural bowl, partially open to the ocean. The waters crash and swirl violently within the bowl. (Inspired by the Devil's Punchbowl, in Oregon.)
A magus must place himself inside the bowl, and allow himself to be battered by the waves. This allows them study Aquam, as if using vis, up to a level of 20. Characters can use one physical characteristic to reduce damage - roll against an Ease Factor of 12. Success indicates that the character is undamaged by the experience. Failure causes the magus to incur a medium wound, and a botch (players roll three botch dice) requires the character to roll on the Crisis table on page 170, although it does not end a Longevity Ritual. Using it more than once a year requires an Aging roll.
An earlier question on the main forum ([url]Build Point costs for Significatos?]) suggested that this should be about 20 BP
<I want a risk of injury for using it, but I'm not exactly sure how bad it should be.>
Unless all of our Magus already have a good way to get to the Island of Delos I think we need a better and larger boat. Sure Paulos have a circle teleport and a fixed arcane connection but that would be 1 warping point each way for everyone except his familiar and him.
Also as ambassador Paulos might need to arrive earlier.
I like the idea of a significato, and this is a clever one, but I have a few observations.
1.) You don't mention how many vis it stands in for. I assume 4, since it's good for up to level 20 study. But it's only 3 botch dice, so I'm not sure.
2.) You don't mention if it's in a Magical Aura (I would assume so.) If it isn't, then it's not as appealing for studying vis. (Maybe so much so that no one would use it.)
3.) I'm not sure that I'd risk death (a rare, but possible outcome on the aging crisis table) or Warping (requiring a CrCo 30+ spell to survive) just to study a few pawns of Aq vis. I'm more likely to just study the vis and not take the risk - or get a tractatus.
4.) Would a Gold cord let you reduce the botch dice (down to a minimum of 1, I assume)? If not, then it's actually worse than studying vis.
5.) We don't have anyone that has an Aquam focus, so I fear that an Aquam significato wouldn't get used.
In the end I think it's a great significato, but I have a strong suspicion that it would just sit there, never used.
We could always just take a regular boat to Delos. There's nothing saying that we have to arrive by magic. We're a very wealthy covenant as it stands. It would be reasonable to assume that we either (a) own a boat or small ship that could take us to Delos, or (b) could easily afford passage on a ship that we paid to stop there. I have to assume that some magi get together to arrange passage from some centralized point to Delos via mundane means. Perhaps some of the covenants with ships arrange to ferry others from major ports to Delos for the Tribunal.
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It's like 4 dice, so you can study up to level 20 in Aquam. The three dice mentioned are for withstanding the waves, not for vis study. You resist the waves crashing on you either with Stamina (endure it), Strength (hold on to a rock), Dexterity (flow with the waves), or Quickness (dodge the worst ones). I'm envisioning that you spend a day in the water, then the rest of the season contemplating the experience. So you don't need to roll for botches for using vis, but you do need to worry about being injured.
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The Goblet is within our Regio, so it's in an Aura of 6.
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For a young magus, the roll isn't going to be bad. We could define it that it's possible for more than one person a season could experience it.
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A Bronze Cord might allow you to reduce damage.
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True, and Aquam isn't a heavily studied form. Not everything has to be completely useful to us. It would be an attraction to other magi who do wish to study Aquam.
Well, you've convinced me. 
I say we take it!
That leaves us with 20 BP more for vis sources. Anyone have a last request for 4 more vis/year?
Currently our vis production is as follows. I note that we still do not have a dedicated source that will maintain an Aegis higher than level 20. Maybe we want some more Vim.
Vis Per Year
Creo: 3
Creo Ignem: 6
Perdo: 2
Rego: 4
Aquam: 2
Corpus: 8
Ignem: 2
Imaginem: 3
Mentem: 3
Terram: 2
Vim: 4
I didn`t say a normal ship can't reach the isle but finding a moving isle is a other issue.
It occurs to me that Daedalus and his brother are likely to go harvesting trees in a level 6 magic regio for wood, which will likely be severely warped for anything even beginning to resemble old growth, and there will likely be a vis source from this (probably extracted before the wood is used). Though this source can obviously cover a large range of vis possibilities...
That does raise a good question, How do all the magi get to Delos for Tribunal meetings? Some will teleport, to be sure, But most won't. How do the rest get there? By boat, we assume. How do they find Delos? It's a small island, but still pretty big by person standards, and I don't expect it moves very fast. I guess I was just assuming that the tribunal authorities (or someone with a nautical bent - Favonius magi, I'm looking at you) provided the necessary details to find it. Then, as I suggested, there are probably a bunch of ships that ferry multiple magi to the island.
We can certainly use our last 20 BP to get an Herbam vis source, if you like. The limit on the vis source may represent that they only harvest a few trees a year.
Just don't flaunt it to Diantha that you're cutting down trees. It'll probably upset her. 
How about:
[size=150]The Old Growth of Therma[/size]
The oldest trees in the regio have been absorbing magic for a long time. This has warped some of them in interesting ways and others in useful ways. Some few have also been infused with vis. The covenant believes that it can safely harvest 4 pawns of Herbam vis without threatening the health of the forest.
Production: 1 He vis/season
That still leaves us short of Vim vis for our Aegis. But I'm sure we can work that out over time.
BTW, I've taken to calling the regio "Therma" for lack of another name. Anyone should feel free to suggest another.
Also, do we have any thoughts on a name for the covenant? Thessaloniki Covenant is a bit bland. We could keep Oikos tou Eleous (House of Mercy), since a number of us are focused on healing. But something else might be appropriate. Also, how much of the history of Oikos tou Eleous are we keeping? It's fine by me if we've fought some diabolists in the past decades. That seems like something we'd do.
Looks good. Rounds out what we need. Therma is an excellent name for the regio.
I'm OK with Oikos tou Eleous, and that we've fought diabolists. We do need to make the covenant a bit older, have it founded at least by 1140 (80 years ago.) Poenitens has been here for almost his entire career.
Well then, that puts an end to our BPs for vis sources. We just need to spend 10 more BPs on enchanted items and figure out the distribution of our vis reserves.
I think we can easily extend the life of the covenant backwards a while. Zacharias has been there a while too. Though I figure he did his apprenticeship elsewhere, so maybe not as long as Poenitens.
So, on the subject of vis reserves, I figured we ought to base it on our vis sources and what we'd have surpluses of. With that in mid, here's a proposal:
Cr: 10
CrIg: 4
Pe: 4
Re: 9
Aq: 4
Co: 21
He: 11
Ig: 6
Im: 8
Me: 6
Te: 3
Vi: 10
TOTAL: 98 (=100 BP because of the CrIg vis costing half again)
I just tossed numbers down and evened them out because it would seem odd if our reserves just happened to be all divisible by 5 or something.
As an alternative, we could take some reserves for things we don't produce, with the idea that we keep some of that around, just in case.
What do people think?