Vokes has been sidelined since he suffered a torn hamstring on his debut for the Robins against Millwall on the opening day of the season.
Millen accepts the 21-year-old may not play another game for the English Championship club, but is keeping an open mind regarding an extension in January.
"I have not asked that particular question yet, but it is something I may consider if the situation is right," Millen told the Bristol Evening Post.
"First and foremost, Sam needs to prove his fitness and make himself available to play Championship football.
"Although he is back in training and close to making a playing comeback, that has still not happened yet and we're waiting to see how he responds after a game.
"If he does get himself into a position where he is ready to play, then we'll deal with that when the time comes.
"There is a chance we may be able to do another deal in January, but nothing is going to happen until Sam is fit and we have spoken to Wolves about it.
"It may be that Sam figures in Mick McCarthy's plans for the second half of the season and we will be unable to keep him anyway. We'll just have to wait and see."