wrote in message
So I'm wondering (hoping) whether I can make the original vent smaller,
as it now only serves the water heater, and all the cold air coming in
from the outside is wasting energy needlessly.

The furnace installer (who was pretty junior as he was young and
screwed a couple of things up and I had to call the company back to fix
the problems with an older repair man) had said to leave the original
vent alone.

I lived in three houses with gas water heaters. They had no vents at all.
I see no reason it could not be reduced a bit.