Perhaps Vadar did let Luke win, but we're not discussing personality traits here - and the evidence is that Vadar lost, horray for him not deciding to pull Luke to bits where he stood, but he still lost.
As for Yoda... No amount of lightsaber ass kicking is going to do any good when you've got to get your ship of choice past the Imperial fleet to get to Vadar, it's all good and well if he could've wizzarded himself off to Vadars bedroom for merry hell-raising between them but he'd have had to get off his rock, get a ship and then get past god knows how many ships/storm troopers before
finally getting to fight Vadar. The nearest Star Destroyer would've blown him out of the stars before he had a chance to get close. So yeah, I can see why not staying and fighting makes him cowardly.. perhaps if he could've E-mailed Vadar and said "look, I want to fight you in a lightsaber duel but I don't want to get blown out of the sky on the way there... deal?" He'd have had better luck

Besides, by running he helped Luke who in turn went out and fought Vadar. Doing far more good than wandering into the fight and getting shot out of the sky would've done. So what he did wasn't weak, it was intelligent.