It's well known that the monster design guidelines don't match up so well with actual monsters - particularly, most high-level monsters punch like a grandma. Vampires, however, seem to match up fairly well.
Defensively, vampires are fine. They have about 140 hp, with regeneration giving them 60 effective extra hp and resistances effectively increasing that by an additional 25%. That gets us a bit over 250 hp, which is on the low end of what you'd expect from CR 13, but fine. Legendary Resistance count for an additional 90 hp, bringing us to 340, which is more like CR 18.
Offensively, they can hit for about 50 points per round if all their attacks hit (multiattack for unarmed + bite, and then again with legendary actions). 50 points is more like CR 7-8. Their attack bonus is a little high for their CR, but within reason. If you include 10 wolves as well, the damage goes up by 70 points (but with a much lower attack bonus), and that gets us as high as expected CR 19. Using swarms instead only gets us about 25 points of extra damage, but that's still enough to get us to CR 12-13.
IMO, monsters summoned over the course of an encounter should be considered part of the summoner's CR, not as independent monsters, and as such be included in the summoner's damage. If you consider the wolves or swarms separately from the vampire's challenge rating, they fall quite a bit, but their strong defense still makes them fairly CR-appropriate.
