It's a great plan. Here's my only issue: why do we need to go back? We got our rears tanned by that vampire. The lesson we SHOULD have learned from that encounter is that Bloodless MacBloodlust is a lot more powerful than we are. Take the hit on the chin, recognize mistakes, and move on. There's a whole big world out there--a world so big, in fact, that we can gain so much experience that one day, when we're firmly set traveling down our paragon paths, we can pay Toothy McTootherson another visit. Any oh my, won't HE be surprised!
My recommendation is this. We take a dose of humility here. The vamp cleaned our clock. Ok, so we know that at this point in time we should be careful around high-powered undead (though when you think about it, I don't think it was much of a revelation, just a confirmation of what none of us wanted to admit). There are a bunch of plot threads we can easily pursue. Here are some of them:
1) I was told by Nathaniel, innkeeper of The Rusty Axe, that the dwarves are digging all over Waterdeep, in places they're not supposed to be. No one knows what they're up to, so it would be pretty neat to find out.
2) We had an evistro mess with the party and that pride-wound has not been accounted for. The demon left the boy's body right before impact of the oh-so-splatter-cool ballista we had made. So, he's out there somewhere. The other question we never answered is why was the evistro possessing the boy? Bairwhin's ritual book had no such ritual for summoning demons, so he did not do it. Then who did? There's a big chunk of the story we're missing there.
3) Leave Waterdeep? I have a large, rolled-up map of Faerun and let me tell you, it is HUGE. Where else do we want to explore?