Change of Target on Mandatory action

Change of Target:
Only usable if this minion is blocked, before block resolution.
Unlock this acting minion , and the action ends (unsuccessfully). This minion cannot perform the same action again this turn.

What happens when a vampire with no blood gets intercepted when hunting and then plays Change of Target?

He unlocks, and then cannot perform the mandatory hunt action again. Therefore the vampire cannot do any actions that turn.

If a minion cannot take a mandatory action (for whatever reason), the he is stuck and can take no action. [LSJ 20010810] (from the general rulings page)