The 29-year-old Welshman suffered a groin injury on the eve of the Black Cats' first match on their pre-season tour of South Korea and returned to action for the club's Under-21 side in a 2-2 draw against arch-rivals Newcastle on Tuesday, playing the full 90 minutes.
Despite his comeback, Vaughan believes that he is playing catch up with his team-mates after missing such a large chunk of summer training and expects a tough task to get back in the starting XI.
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"I finished the season with the injury and, for one reason or another, it didn't get sorted during the summer break. It was tough to have the operation at that time, but I've just got to get back into it now.
"It is tough when you miss pre-season. You come back and the rest of the players are much further ahead fitness-wise.
"You're playing catch-up the whole time so you know it's going to be a couple of months down the line before you're at the same level.
"But the season is halfway through by then. Players are playing for their place during pre-season as well and putting ideas in the manager's head. You miss that opportunity which can be frustrating."
Vaughan scored two goals in 27 outings for the club in his maiden year at the Stadium of Light and hopes to fight his way back into the team soon.
"There's obviously quite a few central midfielders at the club and we've all got different attributes. But hopefully I can get a chance and hold my place in the team."