One of the players at my LGS came to me with this question, and as I wasn't 100% sure of my answer, I promised I'd ask here.

Anson controls a Serene Steward, a 1/1 White Kor Ally, and a Shambling Vent. Noah controls three 1/1 Red Goblin Tokens. Anson activates Shambling Vent, and declares all of his creatures as attackers. Noah blocks the Serene Steward with two Goblins, and the Kor Ally with the third, leaving the Shambling Vent unblocked.

What his question was about was whether he was still able to pay to put a counter on a creature. My inclination is to say yes, but I've seen seemingly straightforward abilities work in weird ways before and wanted to be sure.

Yes, Serene Steward's ability will trigger and allow Anson to put a +1/+1 counter on a target creature, assuming he pays the {W}.

Immediately after combat damage has been dealt, the game checks for any relevant triggered abilities that want to trigger off of what just happened. It looks around and finds that there's a Serene Steward on the battlefield that wants to trigger off of Anson gaining life due to lifelink. So the Steward triggers. State-based actions are checked shortly thereafter and put the Steward into the graveyard, but since the ability has already triggered, it will be put onto the stack regardless of what happens its source afterwards.