Quote Originally Posted by anaximander19 View Post
Ah, I see. I kinda like the idea of having 9 levels of vestige; in 3.5e there were 8, and there are nearly twice as many vestiges in levels 4 and 6 as the other levels, so it'd be easier to shuffle 8 levels out into 9 than it would be to condense down to 5. Plus, it makes balancing the vestiges easier because I can scale them to match the equivalent spell level in terms of power.
That's fair enough if you're set on porting across all (or most of) the 3.5e vestiges. It sounds like hard work to me, but I look forward to seeing how it turns out.

Quote Originally Posted by anaximander19 View Post
You're missing the limit that you can't bind more than your level worth of vestiges. As a 20th-level binder, if you bind 9+9+2 = 20, then expel the 2, you have the potential to renegotiate 8 levels worth of vestige, but you only have space for 2 levels before you hit that cap.
OK, so the cap is on the level of vestiges you can have bound at any time, not on the level of vestiges you can bind in the morning. I can't see any problems with that.

Quote Originally Posted by anaximander19 View Post
That's what I'm aiming for - remember, I'm building this specifically because our party are converting to 5e and we have a binder; I've been discussing with the guy who plays that binder, and that's one of the main things he likes about the class, so I'm doing my best to carry that over.
I've never played one, so I'm kind of curious as to what effect it has on the character and party to be able to bind 9+9+2 one morning and 5+5+5+5 the next. It's very different from how all other classes work, much more variable, but that's part of the appeal of the class, I suppose.

Quote Originally Posted by anaximander19 View Post
Some of the 3.5e vestiges support this brilliantly (off the top of my head, I remember there's a 4th-level that gives Find Steed, and there's a 1st-level that gives dwarf Stonecunning, etc) so the idea of keeping lower-level vestiges for niche benefits is spot on. As for the availability, that's partly why I'm keeping 3.5e's name/title/seal idea - even knowing a vestige's name isn't enough; you need to find and learn its seal so you can draw it during the ritual. This allows the DM to limit the availability of vestiges, and opens up some plot hooks for quests to find the seal of some powerful vestige.
It sounds like you have this all worked out. I hope it comes together for you and satisfies your player, but the signs are good.