I'd actually suggest keeping the vampire but making it more of a non-combat encounter. Sandesyl could be a good source of cult lore. One thing that isn't really played up enough in the campaign is the fact that not everyone in the cult is happy about the new focus on Tiamat (which has, believe it or not, been in the works since the 2e days - the old Cult of the Dragon sourcebook talks about how the church of Tiamat in Chessenta has been trying to infiltrate and take over the Cult of the Dragon almost since its inception.)
I ended up using her in a weird counter-alliance between her and Tharchion Eseldra Yeth. Basically, I decided Yeth was looking to curry favor and/or sent to hunt down the renegade Thayans like the book mentions, but formed an alliance with Sandesyl and Naergoth Bladelord as the means to do that. Essentially, the PC's have wrecked havoc on Severin's plans due to crashing the castle, having the black dragonmask in their posession (I wasn't too keen on the whole "mask is teleported away if Rezmir dies" thing), having the white dragon mask currently 'lost.'
When first formulating this idea, the second book wasn't out yet, and I didn't really know about Yeth either, but she ended up fitting the bill for what I was doing. What I did know is -- at least for the way our group was rolling -- a vampire encounter (even an RP one) was going to be too much happening at once. So I took Sandesyl out with the intention of using her somewhere else down the road (same with the white dragon). Of course, I also ended up changing up the nature of that castle a bit anyway. The PC's basically found dead cloud giant bodies stuffed in rooms and pieced together what happened (cult took it over by force, and was piloting it the good old fashion magical way -- red wizards). Where was a much bigger red wizard threat on board anyway, so between them, cultists, ogres, and 2 boss characters (Rath and Rezmir), it was still fun.
I just wouldn't really recommend going my route for that as a baseline. It's just how mine worked out.