So we can probably assume a few things to help us put Vader in a "dnd like" contest.
- Force Reflexes and Precognition: The Evasion class ability and Foresight spells seem to be a solid rationale here.
- Force Choke/Telekinesis: The telekinesis spell is a good note here.
- Lightsaber: A flaming longsword is probably a decent start, maybe mixed with a "Shatterspike" for object breaking power.
- Probably a high level fighter "chassis" for some base stats.
Meanwhile, we can use the 5e "Death Knight" for Soth, with the addition of things like Power Word Kill.
So it seems to me is neither is going to be doing much "spell slinging" here. Both are going to have advantage to saves (Soth through magic resistance...which is basically the force in this combined universe), and Vader gets advantage on all saves through Foresight. So its going to come down to mostly raw attack power.
Foresight gives Vadar advantage on all attacks, but Soth has a shield for extra AC and has parry for a pretty big AC boost (and we should assume that a magic weapon can parry a lightsaber....just as two lightsabers can parry each other). Both are probably dishing out 3 attacks a round. Soth will also have disadvantage on his attacks. However, Soth can use staggering smites for the first 3 rounds without losing any attack power, adding a lot more damage onto any hits (and there is always a small chance vadar fails, though unlikely).
I think ultimately Vadar is going to get in more hits, but Soth will do more damage per hit (that extra necrotic damage on his attack is quite a bit more damage). The big question is.... can Vadar kill Soth fast enough before he gets below the 100 hp threshold and just gets killed with power word kill?
I think my answer is probably no. If we assume that both are roughly comparable in fighting power.... the fact that Soth can just autokill Vadar at the finish line will see him through.
