I'm trying to put together a spirit summoning mage using only the rules from the core rulebook, TM:RE and the house books. So none of the alternate magical systems in cradle and crescent, RoP:D or Rop:I.
For virtues I've decided to go with potent magic, mostly because I really like it and the bonuses are good, plus everyone wants to purchase your spells, also gone for Spell binding because its a really interesting idea.
What I'm stumped on is what arts should be most important for spirit summoning, binding and controlling. Obvious for techniques its Rego, Rego and a bit more Rego. For forms though, I could go for the elemental forms and be able to work with appropriate spirits (e.g. ignem for fire spirits), mentem allows for ghosts (which I don't want this character to mess with, I already have a ghost focused necromancer specced up) and vim allows for demons and "other supernatural creatures".
I'm envisioning that this character would work with mostly magic spirits with maybe a few faeries thrown in for good measure but ignoring the divine and infernal entirely.
I'm also imagining that he will go with either (or both) hermetic spirit magic or hermetic theurgy in later life.
So my queries, is Vim sufficient for all spirit summoning, binding and controlling, or do I need other forms? Is there any advantage to going for elemental based forms aside from access to all the other cool stuff they get you? Anyone else built a spirit summoner type character and how did you do it/was it practical?
This is a bit unclear I think.
I'll give you our house rule, which I think is also the official rule, but since we explicitly clarified it, it is written down as a houserule:
Vim is enough, but might not always be the best choice.
Vim spells have to be specified according to Realm - thus a Command the Spirit of Magic or a Command the Infernal Spirit, both ReVi spells.
Spirits have Might scores that are aspected towards Arts. You can use these regardless of the Realm.
Thus Command the Spirits of the Night (ReMe) would work on any ghost (actually any Mentem-based spirit), regardless of Realm.
Hermetic Projects pg 111 "Other Realm Ghosts" says:
"If a magus is making a living corpse instead of becoming one, he can decide which type of ghost to summon, bearing in mind that spells and devices must be realm-specific." and "All such spells will still be Rego Mentem spells, but a Rego Mentem spell designed to summon a ghost affiliated with the Magic Realm (Magic Might) will not work on a ghost affiliated with any other realm."
Note this is sort of contradictory from the guidelines given in RoP: Magic (pg 111) "Spells of a specific Form can command any spirit tied to that form. For example, Ignem can command any fiery spirit." I'm not sure if this is sloppy wording or not. The term 'Airy Spirit' could refer just to the Magic Realm, but the ReVi guidelines in the same sidebar refer to 'Airy Spirit of a specific Realm'.
The only thing that's clear is that Wards must be Realm specific, regardless of Form, but make no mention of Realm otherwise (Lay to Rest the Haunting Spirit, Incantation of Summoning the Dead, etc).
Tellus' house rule is a good one. Vim is still more valuable than the other forms, as it lets you control any spirit with just 4 spells (one for each Realm) as opposed to 10 spells (one for each Form that ignores Realm). Vim is also the easiest Form of vis to extract (no virtues required) when making magic items or ritual spells to deal with spirits, and is the Form that determines how much vis you get when doing so anyways. A spirit magus should always prioritize Vim.
The idea was that you're not forced to focus on Vim though.
If you're eg. an expert on Ignem, you can use that knowledge to command spirits of Ignem, be they Fey, Magic or Infernal.
Certainly a non-Vim specialist can leverage his focus to control spirits in his chosen Form, but the OP did ask which Arts to concentrate on as a spirit magus. The other forms aren't useless, but Vim is really good in general and for spirit magic.
Going for the elemental forms has the advantage of dealing with Elementals, which aren't quite the same as Airy Spirits but can be affected by Spell Binding all the same, and you can flat-out conjure elementals as needed (via ritual magic), where summoning spirits often requires an arcane connection and it can be pretty pot-luck. Someone with high Ignem can 'easily' create a powerful fire elemental by creating a large fire and casting a level 35-40 CrIg ritual spell and then use it to sustain a powerful Spell Binding. Someone just focused in Vim doesn't have that option, though they are still likely to be able to find a suitable spirit with enough effort.
Also, elementals have cunning rather than intelligence, so you don't have to feel so bad about locking them in jars to sustain spells, and they can't talk people into breaking them out. In theory you can bind them into breakable objects and have elemental grenades, though it's not exactly a reliable means of attacking people. Elementals can be tamed/trained too (RoP:Magic) rather than just flat-out commanded/controlled.
I can see that for an effective spirit mage, Vim is the way to go. Given that I'd be ignoring the divine and infernal, I'd really only need spells for faerie and magic realms.
However, and although this is obviously a more costly way of doing it, I really, REALLY like the idea of playing an elemental summoner as suggested by John Prins. It has a really nice flavour to it and I could start the character off with only two elemental arts and build on the others in play.
Note also that Rego Vim intangible tunnel and spell sustainment spells also stack rather nicely with ... everything. You should also take another look at Spirit Binding with those in mind, a Spirit Bound sustained sustainment spell is all it takes to only need one version of that spell to sustain all sorts of things long term. Rego Vim + Spirit Master is a concept I'm rather fond of, with a wide variety of different takes on it really.
Also, bear in mind that a particular vulnerability for you is the spells you use to control your spirits/summons. A very efficacious way of preventing them from being targeted is a Perdo Vim concealment spell of a good level. They can't target your control spell if they can't detect it. It also gives you deniability if you should desire it. ( 'I" didn't summon that Faerie Knight. I have no idea what you are talking about. Faeries 'have' been known to do things on their own volition, from time to time... )
I strongly recommend some good/decent levels in the Realm Lores, so you realistically know a bit about what you can summon and what you can feasibly ask them to do.