Vaughan is out of contact next summer and with no sign that he will sign a new contract the club look set to cash in on him instead.
The move now hinges on Vaughan and the player will fly out this weekend to discuss the move with manager Chris Coleman.
Gradi told www.crewealex.net "We have accepted a bid from Real Sociedad for David Vaughan. We rejected their first offer but they came back to us and upped to £300,000 and although we feel a David Vaughan at his best is worth more than that, he is out of contract in the summer and could go for nothing.
"Vaughany may take a calculated risk and try to further his career through a different route. As a Full international, he would be thinking that he should be at least a Championship player and with Chris Coleman as the manager over in Spain, the Welsh connection is there.
"Who knows what he might do and we are completely relaxed with it. If we would like to go then that is fine. If he feels it isn't quite right then we're fine with that too. The deal could fall through, but he is going over there to talk to them and the ball is firmly in his court.