Vokes injured his hamstring on the opening day of the season and is due to return to Molineux at the end of December having played just 13 minutes of football for the Robins.
Millen said: "I have not asked that particular question yet, but it is something I may consider if the situation is right.
"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."
Source: PA
Source: PA