You're focusing on the wrong thing here.
Yes, Guga and Sabatini had more supination than Vilas, but not as much as the current WTA and ATP pros.
Plenty of photos of both the above pros on the finish, and an open racket face. Depends on which shot was hit and how high the ball was.
Vilas did finish openfaced on almost every topspin backhand, but he also used a very strong grip...prolly like Sabatini and Guga.