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….
While in mist form, the vampire can't take any actions, speak, or
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?