Turn To Mist (Vapor Form) With Prone & Grabbed

My PCs encountered a vampire. The barbarian successfully trips the vampire and then grabs her.

Que the vampires turn and I am thinking about using the vampires special vapor form, Turn To Mist to get rid of the conditions - there is nothing preventing the vampire using it, it's not even manipulate.
But some of my well-versed-in-rules players seemed to really dislike it, mainly the "free" getting up (which I countered with the polymorph trait, you are getting magically reformed).

In my book it still costs 2 actions (physical to gas then gas to physical) same as removing the grabbed and then Standing up would, it does bypass rolling the fortitude check and any reactive strikes though.