There are a number of cases in D&D where the exceptions to a rule are not stated.
This is where the "specific beats general" principle of interpretation becomes relevant. "It can use its action to polymorph [...] back into its true form" is a specific rule; "the vampire can't take any actions" is a general rule. So, a vampire in mist form that is not in sunlight or running water can use its action to shapechange out of mist form.
Furthermore, there is wording in the "Misty Escape" entry that implies that vampires in mist form are usually able to change back to vampire form. As laid out in KorvinStarmast's question "To Kill a Vampire in Mist Form":
Misty Escape. When it drops to 0 hit points outside its resting place,
the vampire transforms into a cloud of mist (as in the Shapechanger
trait) instead of falling unconscious, provided that it isn’t in
sunlight or running water
This sentence establishes that a vampire turned into a cloud of mist by Misty Escape follows the same rules in general as a vampire turned into a cloud of mist by Shapechanger.
While it has 0 hit points in mist form, it can’t revert to its vampire form, and it must reach its resting place within 2 hours or be destroyed.
If vampires in mist form could not revert to vampire form under any circumstances, this sentence would be redundant.
