The other day I was trying to compare the Summoning ability with the Hermetic spells for summoning and controlling supernatural creatures, but I discovered that the Rego Vim guidelines aren't in the core rulebook. Are there guidelines in another sourcebook?
I believe there are some in RoP:Infernal, regarding summoning demons; there definitely are some in RoP:Magic regarding summoning spirits.
Long answer - everywhere. Short answer - well, The Mysteries: Revised Edition is your best bet. Essentially though, it just dupicates the rules from the Core, with Rego [Form] 5 to control and 15 to summon a spirit. Interestingly, the rules imply that the spell used is not realm aligned, but that unless an arcane connection to a specific spirit (rather than, for example, fire as a connection to fire spirits in general) is used, the spirit that appears is determined by the realm in which the spell is cast, with demons tending to show up in Divine auras.
TM:RE also has rules for some other fun spirit stuff, including Theurgy. RoP:M doesn't have new rules for spirit summoning, but does give lots of information on spirits in general and rules for designing and running them. HoH:S has rules for Hermetic Sahir and dealing with Jinn, and RoP:I has rules for non-hermetic summoning, and some of the nasty things Infernalists can do, though frankly I find the idea that binding spirits is innately infernal rather simplistic.
The rules are, however, rife with conlficting statements, so Your Saga will inevitably Vary.
Oups! This is right.
(4 months later)
Vortigern? I don't think your character can apply his spirits MMF to Goetic Arts
The (Form) spells have certain advantages over the Rego Vim spells - Rego Vim does need to be Realm specific (but are wide ranging and general vs Might/spirits), while Form-specific spells are more directly connected to their class of target, but more limited.
Strictly speaking, if you use (say) "a fire" to summon any fire-spirit in the vicinity, you don't get one from the Realm you are in - you get any spirit in the area... Granted, though, a Magic aura will usually have Magic-based fire spirits, and a faerie Aura will have Faerie-based fire spirits... just a note that the spell summons by the link to fire, and that a spirit from a foreign Realm might just be there...