.. you cannot continue the "Reinforcement" action by any means (e.g. "Form of Mist" or "Ambulance") after it was blocked. The reason is that "Reinforcement" per its own card text is removed from the game even if blocked, and a minion can only continue an action (when blocked) if the action card is the ashheap at that moment. Another card that's not moved to the ashheap when blocked is "Swarm".
Reference: VtES Usenet Newsgroup or the VtES Rulings webpage.