Limits to Vampire thralls?

In 3.5 it says that a vampire can only control twice the amount of HD worth of spawn. It goes on to say that if a target has more than 5HD they become a vampire, not a vampire spawn, and that any vampires they make are under the control of the original vampire sire.

My question is this: is there a limit to how many vampires (not spawn) that can be made?

Example:

Nosferatu is a 12 HD vampire.

He turns Wendel Wallace III, a 12 HD fighter and Xyla, a 12 HD sorceress for a total of 24 HD.

They become vampires. Can they now bite another two 12 HD people and essentially have an infinite chain of high level HD vampire slaves?

I feel like I must be missing something here because there has to be a limit.

I'm aware that spawn can't create more spawn, but if I'm reading things correctly, then these vampires that are 5 HD or more aren't actually spawn so they're not bound by that limitation. Still, I'm having a hard time figuring out how this doesn't make it so that one 6HD vampire can control an unlimited number of 12HD vampire by simply having the first 12 HD vamp turn two more 12 HD vampires.

The only way that I can make sense of this is that if the original Nosferatu turned Xyla and Wendel Wallace III (for a total of 24HD, which is his maximum amount), then any vamp they turn after that will be free-willed because we're only going off of the original vamp sire's HD, not adding the HD of the enslaved vamps. Is that right?

Sorry for the confusion and any help would be greatly appreciated by me (but probably not my players).