side with the vamps. from a powergamer standpoint you dont get a significant amount more of XP siding with the thieves and you get way more magic items to use sell (mostly +1 swords but hey, vampires dont drop anything).
you can do the renal/maevar quest stuff before choosing a side, but not after if you pick the vamps so get that out of the way.
p.s. i challenge any of those that are saying "you get more XP siding with the thieves" to show me a tally for each quest. i think that's a load of crap

.