Page 297 MM, (emphasis mine):

> 
> **Shapechanger.** If the vampire isn't in sunlight or running water, *it can use its action* to polymorph into a Tiny bat or a Medium cloud
>  of mist, or back into its true form....
> 
> 
> 

Shapechanger continued...

> 
> While in mist form, *the vampire can't take any actions*, speak, or
>  manipulate objects.
> 
> 
> 

Since it takes an action to polymorph back into its true form, and a vampire in mist form can't take actions, does this mean that a vampire is stuck in mist form once he polymorphs into it?