Vader was so broken down in Empire that Luke stood a chance in a fair fight, even though the real argument is that Vader was playing with Luke. Seriously it looked like Abe Simpson waving his cane around in the dark.
If Luke died in Empire, or fell to the Dark side, in another 5 years, by the time Leia was trained to Luke's level in Empire, Vader would have just been some dude in a floating iron Lung in####ingcapable of light sabre dueling.
Vaders injuries would have more than halved his life expectancy.
Sheev was 90.
Exactly how long did the Emperor and his Apprentice have left to rule over their Empire?
5 years? 10?
The plan should have been to wait out Palpatines life span, and then laugh as Annie cocks it all up, if he even manages to take power, since "Apprentice" doesn't actually mean second in line for succession considering how Tarkin pushed Darth Vader around in New Hope.
PS
You must know about these ancient alternate universe comics.
https://en.wikipedia.org/wiki/Star_Wars_Infinities#Star_Wars_Infinities:_A_New_Hope
In the first one, the Death Star survives the battle of Yavin 4, so Vader begins teaching Leia about the Darkside.
PPS
It's possible that a private schoolgirls education, including polo, zero G fencing, field hockey gymnastics and droid-skeet shooting, y'know games that rich people play that easily transfer into war skills, that-Leia could have been way ahead of her small animal shooting hillbilly brother as an artful warrior.