Bucky is not comparable to Darth Vader for me - Darth Vader was a protege, a student of the Emperor's who chose the dark side. Bucky as Winter Soldier is just a blunt instrument. He has no feelings/memories/choices of his own - those were all taken away from him right along with not just his identity, but also his status a a person. Bucky Barnes never went to the "dark side" - he was basically erased from existence and his body was used as a weapon. I thought that was made very clear in the brain wipe scene. Heel/face turn doesn't work for Bucky at the end of TWS, especially as a comparison with Darth Vader.
Are we talking the MCU movie version or comic version? In the comic, his memories were wiped, but it was like he started a new life in Russia being their assassin. He had feelings and emotions, which is how he and Black Widow fell in love, back when she was working for the Russians. Cap used the cosmic cube to make Bucky remember everything, that how he became good again, but he never forgot his relationship with Widow and they became a couple again (until her memory got wiped).