Isabel was "an inactive Lucian" until the clue hunt, so that would likely mean Vikram wasn't a Cahill ... but you're right, she's totally a Vesper (it's in her maiden-name for crying out loud!). Maybe one of her parents is a Cahill (Hollingsworth) and the other is a Vesper- Oh! You know what'd be cool? If we got to meet Ian and Nat's Gran or Gramps and they, like, lure Ian over to the Vesper-darkside somehow (but in the end he doesn't really go). It'd be cool conflict, and it'd add a reason to why Dan and Amy can't exactly trust him 100% (until some big emotional confrontment happens and things go down).
Wow ... actually, that just sounds like really bad fan-fiction to me, but whatever, I'm still super psyched to find out what really happens!!! :D