My party is currently level 12 with a monk, bladesinger, and a spellsword (an arcane paladin type homebrew class we found). They pissed off the lady of a town, who happens to be a vampire, and she keeps sending her spawn after them to kill them. This happened so early in the campaign that the appropriate levels for her and her minions were a little too much for them to handle at the time.

Now, the 2 head vampires she keeps sending are way too easy for them to defeat. They are a mind drinker vampire and a vampiric priest of osybus. I need a good and believable way for them to increase in power outside of just adding more hp or ac, or just adding more lesser vampires (I have tried that and they walked all over them as well). If there are creatures out there that I can substitute and reskin, that would be ideal.

Any help of advice would be helpful. Thanks!

I think with the Vampiric Priest your best bet is to have him attempt to fulfil his dream of becoming a Lich.  Perhaps he tries and fails, becoming a Boneclaw or perhaps he goes about it another way and becomes an Eldritch Lich, that would suit the character thematically.  

As for the Mind Drinker, perhaps he drinks the mind of a particularly strong magic user and gains some decent attack spells (I mean, heck, he could just learn some more).  You could have the party witness it, or involve it somehow in your plot.

Honestly your party is at a level where even the vampire patron probably won't scare them in combat either. If this happened early in the campaign, maybe it's time for them to finally track her down and take her out, so you can move on to bigger threats and not have to needlessly constrain yourself as the party grows. 

 Vampires are CR13 depending on how optimised they are even a party of 3 should not have any problems taking her down. Adding her two minions might make a challenging fight but you know better than me what the party can cope with.

The party may or may not allow the vampire to misty escape if they do not you can always have her come back for revenge.

I gave both her minions misty escape as well so they have been defeated multiple times. I also want to keep them weaker than their master. They refuse to find out how to properly kill a vampire though. I guess I can just make them a nuisance until they figure it out. A pest that just keeps coming back.

I could also have the minions ascend like Holton said, if the main vampire is killed and they are not. Gain power to avenge their master or pursue their own goals.

nosferatus can vomit blood. maybe these two vamp leutenints have the blood voit thing, or be buffed with hemomancy spells? some kind of blood chalice that tuns into a blood sword? the posiblities are as endless as they are bloody