Will you please post what level/class/race (other pertenant info on each PC) and what experience the PCs have had with vampires in the past?
{I have a feeling that might help support an argument one way or another in this case.}
Again, pretty much no one is disputing that your DM was acting improperly based on the information we are being given - we are only saying that 2 wrongs don't make a right and this is a rules forum.
Failing a save against an illusion pretty much leaves you convinced that it is not an illusion but in fact real - continuing to try to prove it is an illusion goes against any logical argument that a player is using only PC knowledge and not player knowledge.
{I have a feeling that might help support an argument one way or another in this case.}
Again, pretty much no one is disputing that your DM was acting improperly based on the information we are being given - we are only saying that 2 wrongs don't make a right and this is a rules forum.
Failing a save against an illusion pretty much leaves you convinced that it is not an illusion but in fact real - continuing to try to prove it is an illusion goes against any logical argument that a player is using only PC knowledge and not player knowledge.