If a character with a virtue that affects casting, writes a Casting Tablet, does the user of this Casting Tablet get the effect of the author's virtue for free, even if they don't have the virtue?
eg If a character with the Mercurian Magic virtue writes a Casting Tablet for a ritual spell, does the user of this Casting Tablet get the ritual Vis discount, even if they don't have the Mercurian Magic virtue?
Or Silent Magic, or...?
Can any Gifted character who can read a Casting Tablet use it, even if they are not opened to Hermetic Magic?
The section in Covenants is quite brief, as if the author wasn't expecting mass use of them. Bearing in mind what I've read generally, I would say:
No, when you use the tablet the caster is using their Gift to create an effect specified by the writer. In fact, if the author puts restrictions on based on their Virtues, that would make it harder to use. Covenants p90 says the exact number and type of pawns of vis used must be defined, so a Mercurian magus could specify Conjuring the mighty tower using 4 pawns of Terram vis. Your wizard without this Virtue can't make the effect work with only 4 pawns, so it fails. This would mean a Waster of vis could only use ritual casting tablets written by someone with the same flaw. Likewise, if someone with Silent Casting writes a casting tablet which specifies the spell is to be cast in silence, your wizard without the virtue can cast it but would get the -10 penalty.
As you need to have your arts opened to Hermetic Magic to create any sort of Hermetic effect, I would say no. If you disagree, you could have a Mythic Europe where hedge wizards can steal a casting tablet, attempt to cast it just using Stamina + Aura + stress die (as their arts are zero) and see how badly wrong it goes.
I am reasonably certain that can't happen.
Every wizard invents their own spells and casts them with their own quirks (the author's Casting Sigil is always in it). The casting Tablet contains the information to cast a single spell with only a single set of parameters. Doesn't contain enough explanation of how it works (ie theory) to aid in inventing that spell, or even to tweak it to cast it with a different set of Parameters (eg more Penetration), so I doubt a Hedge Wizard could get any Magic Theory out of it.
The forum consensus seems to be that you can't even take advantage of any spell casting Virtue the author possesses.
What I do see is adventurers coming across a mysterious book, and if the socially awkward intellectual can read it then they can cast the spell written on that page/tablet as they read it.
I personally think that the number of pawns used is fixed when writing the casting tablet as if the writing magus didn't have any Virtues/Flaws changing the number, and then that number would be adjusted with the caster's Virtues and Flaws. That would mean that a magus with Mercurian Magic writing a casting tablet for Conjuring The Mystic Tower would write it as using, for example, 3 pawns of Terram vis and 4 pawns of Creo vis. And when casting it, he would get the usual discount of half the amount of vis for a Ritual, meaning he can use 3 less pawns of vis, substracted from those specified in the casting tablet's vis requirement.
My reasoning for this is that the magus writing the casting tablet writes the spell as it is (as he would invent it, so to speak), and those Virtues and Flaws that adjust vis usage apply when casting the spell, not when inventing it.
Then again, I must admit this is quite close to a House Rule instead of an interpretation of the RAW, so as usual YSMV. But you might give that idea a spin.
I tend to assume that the writer sets the amount of vis that is used in the ritual freely, so a Mercurian could set it at the standard level rather than their cheaper one, and someone who knows that the tablet is likely to be used by a Waster of Vis can compensate accordingly.
Of course, the downside is that everyone using that tablet then has to pour in the increased vis amount.