vampire the masquerade – What happens if a character’s declared action becomes impossible?

The action is wasted, at least as it was intended

The major basis for that statement is that combat rounds in V:tM are three-second slices of time with simultaneous actions. The initiative order is used both to make that more manageable (it’s hard to track and interact with 10 characters operating at the same time!) and to simulate the advantages held by characters who are faster than others. Consider this excerpt:

Although you declare your character’s action now (this is the Initiative phase)
(including stating that your character delays her action
to see what someone else does), you wait until
the attack stage to implement that action. At this
time, you must also state if any multiple actions will
be performed, if Disciplines will be activated, and/or if
Willpower points will be spent. Characters declare in
reverse order of initiative, thus giving faster characters
the opportunity to react to slower characters’ actions.
(V20 Core Rulebook, page 271)

Emphasis mine. It is explicit that the purpose of declaring actions in reverse initiative order is to allow faster characters to undercut slower characters’ efforts. It doesn’t fit too well with that to allow a slower character to declare an action, have it thwarted, and then freely choose a different action all in the same 3-second window.

Portions of an action may still be possible, as in the “intending to run through a door which is suddenly closed”. You may not be able to run through the door, but I would almost certainly rule that you still run up to it before it slams shut in your face.

Narration is important. This is a game under the Storyteller system! A situation like the one described in the question, while not too common in my experience, allows for tense and exciting encounters.

When two vampires are fighting in the Deadlands, one doesn’t disappear through the Shroud back to the living world and then after a pause their opponent starts shooting at where their target used to be in frustration. It all happens at once, and the disappearing character acted just a hair more quickly than the other one. The narrative-focused Storyteller system is probably better served by something like:

Lucretia levels her pistol at Augustus and pulls the trigger three times, with three precise, controlled motions. But as the bullets fly towards Augustus, he seems to fade away. In the fraction of a second it takes the bullets to reach Augustus’ former position, there is nothing there to hit!