Hereford were unable to capitalise on their numerical advantage and Marc Richards added a second for Vale before Bulls defender Ryan Valentine was also sent off for a second yellow.
He received his marching orders for his part in a melee involving several players, but Trewick said: "He looked unfortunate to me. There were a lot of people pushing and shoving and it must have been tough for the referee to pick just one player out.
"It was a disappointing end to a disappointing day because we had enough of the ball to create the chances to get something from the game. We did have a couple of chances but we didn't take them.
"We certainly should have defended better for Vale's first goal, which should have been avoided. That sort of thing has happened a few times now and is leaving us with an uphill battle. Giving the opposition a goal also gives them a lift and changes the game."