Yoda was old enough to die of natural causes prior to the main portion of Return of the Jedi, and he'd already lost a 1v1 fight vs Vader's master Darth Sideous back when he was still physically fit for battle.
Quick judgment call: Had he fought Vader back when he first became Vader, he'd have won. Had he fought Vader in the Episode 4+ era, he'd have lost.
Edited: daishain on 1st Jan, 2016 - 1:42am