Rules As Written
These abilities are very poorly written. Shapechanger (as quoted in the question) allows the vampire to transform into a cloud of mist, but explicitly prohibits it from changing back by denying it the ability to take any actions. Woops... blood sucker is stuck forever!
Rules as Intended
From the first paragraph, it's pretty obvious they writers wanted the vampire to be able to assume mist form by choice. The "unable to take actions" should have read "unable to take any other actions"; that would have still allowed it to use Shapechanger while in mist form and retained the fact that a cloud of mist can't really affect the world around it.
The broader "unable to take actions" clause should probably be part of Misty Escape, and is in part. During Misty Escape revering to vampire form outside it's resting place is explicitly prohibited, but turning into a bat isn't (only the no-actions clause of Shapechanger stops that.
Vampires are a hot mess - both Shapechanger and Misty Escape are written poorly and in dire need of errata.
The other answer tries to claim specific versus general, but all the issues are in the same ability, Shapechanger. An ability can't possibly be considered more specific or more general than itself. This isn't a case of specific versus general, it's just bad writing, a mistake.