Ellen Fence uses her special to attempt to enter combat with a tapped vampire.
The tapped vampire untaps (because his Methuselah burns an "Underbridge Stray" he controls ^^).
No block.
Action "fizzles" because there is no legal target for the action.
So weit, so dumm gelaufen bzw. cool gespielt ...

Nun stellte sich die Frage:
Is this action successful, unsuccessful, or does it just end?
Answer (by Regelguru Pascal Bertrand):
The action is successful, since it was neither blocked nor was there card text saying the action ended unsuccessfully.
The action still "fizzles" since there was no legal target.
So, Ellen Fence could NOT have played "Zephyr" (to untap after an unsuccessful action), since the action was successful.