I played weekend tennis for nearly thirty years beginning mid-'50s. For deuce game beyond 40 we used the short terms "ad in" (server ahead) and "ad out" (opponent ahead,) or "advantage [name of player]." Used to watch college tournaments (when in college), and TVed tourneys on a regular basis. I cannot recall either the term "van" or "vantage" was used. Perhaps it is a localism where Backwoodsman or Michael are. Or maybe "ad in/out" is the localism in Southern California.