From the rules themselves, there is absolutely no way to tell. They are ambiguous.
James Jacobs, who is Paizo’s creative direction (i.e. not necessarily directly responsible for the rules) has commented on this question, however:
How does Vital Strike and the like work with slow-firing weapons, if at all? Does the full round action requirement mean you are technically not using the attack action so Vital Strike cannot apply?
It doesn't Vital Strike requires its own action, and with a slow-firing weapon, the attack uses up your available actions so you can't Vital Strike with it.
So there’s the most official answer available, though it is fairly lacking. It’s written as if this is what the rules themselves already say, when in reality they’re ambiguous. Ideally, Paizo would publish an errata clarifying the situation, but that’s unlikely to happen.