Vampire Spawn and other vampires are solid minions thematically. As are other intelligent undead and literally anything susceptible to mind control, given the nature of a Vampire Antipaladin's save DC on Dominate (DC 27 or so).
And just being an Antipaladin does a lot of your work for you in terms of being an effective boss monster. A Vampire Antipaladin is terrifying.
At Level 16 (and thus CR 18, a fitting individual villain CR, especially since I'd round the encounter out to CR 19 with minions), he could easily have 28 Charisma (to go with a 28 Strength, 16 Dex, 16 Int, and 12 Wisdom...assuming a headband of Charisma +6, and a Belt of Strength +6) and thus 236 HP and saves in the stratosphere (+19 Fortitude, +17 Reflex, +20 Will, all before adding a Cloak). With Dex 16 and +5 Mithral Full Plate, a +4 Ring of Protection, a +4 Amulet of Natural Armor, plus a Dusty Rose Prism Ioun Stone and a Jingasa of the Fortunate Soldier he can manage an AC of 43 (47 vs. his smite target), and with an Unholy, Conductive, +5 Greatsword and a source of Haste (Boots work, or a minion who can buff) +26/+26/+21/+16/+11 for weapon damage plus 33 (plus 2d6 more if Good). And that's before Smite (which adds +9 to hit and +16 damage) and Touch of Corruption (which you can toss on via Conductive, and adds 8d6 damage plus your Cruelties). A Quick Runner's Shirt is also cheap and excellent for melee villains. He might even work better as an archer, actually...but I'd go melee for effect, and to give the PCs a chance to escape if they need to.
It's just a crazy combination.
just reading this gave me a shudder of fear, and im no where near the campaign...