Vent away. Sounds like something happened to really shake him up and he was taking care of himself, which is good modeling, and taking care of you, knowing he wasn't in the right shape or mind to be there in the way he knows he needs to be there for you.

If he had gone ahead with the session and fumbled through it not really being present and fully attentive to you, would that have been better?

Sorry it happened. Go right ahead and vent away. Just remember he was doing what in his best judgement was best for himself AND for you at the moment. That IS being there for you, just not in the way you would prefer.