Why would he vow revenge? They both knew it would be an empty threat. Revan defeated him and he realized he had failed in the end to take Revan's spot. He probably realized it was pointless to swear revenge he could never carry out as he was dying and used his last breath on something else. He didn't exactly say "You did everything, I just stole your power" either.
Then he should avoid defeat, he had the Star Forge and Bastila with him and he still lost against his master, that sucks.
Yes. I cannot deny it any longer. You are the one who deserves... who deserves to be the Dark Lord. You were the one who found the first Star Map on Dantooine, Revan, and it was you who led us on our quest for the Star Forge. I only followed in your wake. I tried to usurp your rule, to steal the title of Sith Master from you. But now I understand. The destiny is yours Revan. Not mine. You... you are Darth Revan, Lord... Lord of the Sith. And I... I am nothing.