That sounds better than my idea to make a class of it.
By my understanding Rituals are designed so that they are useful to cover gaps in a party... not got a Rogue? Use Knock, otherwise just get the rogue to do it, it's quicker and cheaper.
Binder rituals could do the same thing for combat. You do a 5 minute ritual and get some of your powers replaced. Each Vestige can have 2-3 powers (at-wills, encounter, daily or utility) that produce a certain role.
Got no wizard and you're about to fight a hoard of minions? Better bind a vestige that lets you gain some controller powers.
You are a Wizard and you're going to fight a single big boss... get some striker powers.
i.e. you do the ritual to fill in for combat roles you may be lacking in, much like normal rituals work in non-combat.
To make it useful you may need to call the vestige in mid-combat then perform the ritual to calm it after the fight is over (or be cursed).
This sounds cool. Much better than having a Binder class.
However, I almost feel like Vestige Pact Rituals, being so flexible, should be separated somehow from other rituals. Having Diplomacy as their key skill is a good start. But what about requiring a separate feat, instead of Ritual Casting, to allow access to Vestige Pacts?
A nod to a connection between Warlocks and Vestige Pact Rituals would be nice, too. Maybe the prerequistie for the Contact Vestige feat could be, "Ritual Casting feat OR Warlock class."