Vader commands a superior connection to the Force which affords him nigh-infinite reserves of energy. Rhea and Vestara seem very skilled, but I would submit that Vader's energy surpassed his son's even at the height of Luke's powers -- so they won't be tiring him out anytime soon. Thus their only avenue for victory is to outmaneuver Vader, which is equally unlikely; Vader was able to defeat the likes of Cin Drallig and Count Dooku.
Vader wins.