Nah, V'ger fired at them even with the shields down. None of what they were guessing was correct: it was the fact that V'ger wasn't receiving the linguacode that is why it fired ("not intelligent, might as well digitize it'). Decker wanted to hesitate and not go into the cloud. Decker suggested firing maximum phasers the break the tractor beam. None of that would have gotten them into the position to save the day. In fact, when the chips are down it's Kirk's bluff ("the carbon units know why the creator has not responded") that gives them the opportunity to figure out what V'ger is. Neither Spock or Decker would have thought of that. Kirk saves the day, if not as directly as usual.
It's even a nice little touch that original skipper Robert April is on board for that final episode. Even if they didn't know it at the time, in a lot of ways "The Counter-Clock Incident" certainly serves as a much better bookend to TOS than "Turnabout Intruder". 
