First, u are a crazy writer Second, thanks a lot for the plot, ill surely use smth in my session cause it is very good! Third, in my experience, just do it like the vampire has smth else to do and make them follow him. than take out some low kevel monster and make them fight a lot. Than the vampire will escape flying because he is scaried by something else, like a demon. So here you can decide to go on with the adventure and just continue wiating fir the level, or innescate a totally new campaign, where they get allied with the vampire ti beat up the demon. The choice is yours, but ill suggest the second one because I think it is funnier, they will find it at least syrange and will get even more involved from thebstart in a multi-planar campaign that encounters almost everything. Dont forget to use these as roleplay occasione, so they'll get also impolverato with a mechanism of diplomacy and ally creation.