It's worth remembering that a full vampire has Stealth +9, regeneration and up to 150' movement per round. They have the potential to be incredibly annoying hit-and-run monsters. Imagine the vampire popping up every few rounds, charming another PCs, and then escaping with legendary actions.
I've used a full vampire twice. The one time, the party monk stunned it with Stunning Strike repeatedly, and that was all she wrote. The other time, it was a semi-friendly vampire whom the players opted to negotiate with instead of fighting, and it sort of allied itself with them for a time (causing long-term problems in the process) before getting killed by a dragon. I suspect that without a monk, the fights would have gone fairly poorly. You need a way to lock it down, and that means stunning/paralyzation/grappling is mandatory unless you have a damage nova so huge that you can kill it in a single PC's turn, which frankly you probably can't, at least at level 7 and 8 where my players fought it. I suppose at level 13 you probably could: GWM Polearm Master Mounted Combatant Devotion Paladin with Str 16 and Cha 18 plus Sacred Weapon active for example could be doing avg.4.10a?d10+13+6d8=157.82 on average in a single round, or avg.4.8a?d10+15+6d8=179.40 on average if he had Magic Weapon IV active. (Plus, the vampire's Charm gaze won't work around the paladin.) That vampire would be in for a nasty surprise if it gives the paladin even a single round to prep.
But then, by level 13, a CR 13 monster is no longer supposed to be a real threat--it's just a Medium encounter. For lower-level characters, a vampire is pretty scary.
