gladiusdei:
in my experience it is very hard to mix the two, that's why this game initially died off. Too many players with their own agendas that did not want to work together in the storyline.
So basically, it would either be a storyline with occasional chances to do your own thing, or a full sandbox where the players decide the goals. It won't be both.
To that end, it may behoove you (just a suggestion) to declare what you want from characters. IE Why did these kindred, this corterie, get chosen for XXXXX. What hook or shared interest could/should they share. Do they each have a "particular set of skills", are they all the same Clan, do they share a connection to an NPC that pulls them into things...?
mind, I'm totally in for story driven :)