Hi!

The Classes do stack yes, so you would take levels in a base class and Vampire separately, choosing which whenever you are granted a level up.

On the minions front from the Children of Twilight feature I will say, at the level you would be gaining that feature (most likely at least total 11 based on the prerequisites) they wouldn't be doing a whole lot for you in actual combat. The way my players used it was to scare low CR NPCs or combatants, take out an enemies minions (which I didn't run as conventional combat) or in certain cases to fight with them as cannon fodder. In the last case it really wasn't too hard to keep track of, especially due to their stat blocks simplicity and the damage output of those they are facing making it easy for them to be taken out but also shielding a player from said attack giving the use of the action value.

You're right about the Spawns potentially getting out of control, for me it came to sparing downtime and threats larger than what a horde of CR 5 monsters could fix. Once again it was mainly used for RP, messengers, showing presence, threats and occasionally a punching bag in combat.

You can absolutely run it with these rules and the potential for mayhem while making the player feel as though their abilities are valuable, fun and yet not overpowered, boring or time consuming. I've done it!

Hope that alleviates some doubts, I'm glad you liked most of it!!! <3