This vampire guy really has no issue giving them all the information regarding himself and his plans. I guess that he considers them to already be dead but that's just not prudent. Moreover, if he believes that the master must be more formidable than the squire, shouldn't he be deeply concerned about his current situation since he's been having a lot of trouble with a mere squire.
I don't understand why a demon lord candidate wants to shapeshift into a priest and go to the royal capital. I also don't understand his plan to start with. He might be able to replace a priest, but wouldn't a delegation that shows up without an escort of knights be strange? It seems to me that he should have attempted to sneak into the tent while the knights were distracted in order to replace one of the priests.
I wonder if he'll ever realise his assumptions were incorrect and escape (or become their traveling companion, though I'm not sure why a demon lord candidate would do that) or if he'll be killed by Mara.