So, I had an interesting session tonight where the group pretty much Leeroy Jenkinsed their way to the Morrowfall.
I had Vahnwynne ambush them in area B20 as someone else had suggested in the What Lies in Dust thread. After quite a fight they reduced her to gaseous form and she fled under the iron doors and down to the Amber Arca. The heroes all drank potions of gaseous form and followed her, at which point they were attacked by the vampire spawn which tried to dominate them and force them to solidify. The barbarian / fighter ended up fleeing down the hallway and grabbing the Morrowfall. Just after this the ranger / sorceror used her scroll of sunburst which destroyed all the vampire spawn (but blinded the other 3 players). This is where we ended the session.
So I'm presuming that Jair and the Mazeflesh Man aren't going to just wait in their rooms for the PCs to destroy them. I'm thinking that the Mazeflesh Man will come out and attack the party pretty much straight away, while Jair will be sneaky and try to ambush the PCs before they leave Delvehaven (possibly at the staircase that leads up to the groundfloor).
Does this sound like it makes sense?
Well, I would say that the 3000gp scroll of sunburst is supposed to kick ass, and remember that your druid has a 45% chance of failing to use it (assuming she's druid lvl 6, and that she would get the spell at lvl 15).
It makes PERFECT sense that Jair and Mazy would attack them straight away, maybe even together? The party just made a helluva lot of noise (I'm just assuming sunburst makes a bang, it seems appropriate). If your party facerolled over everything else, maybe the two badasses together would be a more challenging fight? Personally, I think Jair needs some backup, I can just see my party (who will be in Delvehaven in about four sessions) just destroying him with trips and other "single target" stuff.
A few things to keep in mind:
From your description, Jair and Mazeflesh don't start with line of sight to the casting of Sunburst, though they definitely would see the bright light flash through any cracks in the doorways leading to their areas. Without knowledge of the PCs possessing a scroll of Sunburst, I can see the Vampires assuming that the PCs must already have the Morrowfall and furthermore are either in the process of escaping with it or have come to use it to wipe out the vampires. In either case, I could see a knee-jerk reaction to the flashing with an almost mindless rush towards the PCs. I would say that Jair and Mazeflesh would not have any buffs up in their rush to stop the PCs but they would definitely move to pursue them. Since the vampires dislike working together, it might be possible that Jair would leave the fight after a couple of rounds to go and Buff himself up before returning, especially when he realizes that the Morrowfall isn't going anywhere yet.
Oh yeah, 3/4 of the party will at least be immune to the Vamp's dominate gaze, though being flat-footed against repeated energy-draining slams sounds like sure death for one PC. Maybe the point for the PCs at this time would be to escape with the Morrowfall and fight the vampires another day? Hopefully they entered Delvehaven during the daylight hours so the vamps can't puruse them..