Would really need to see your logs to see if it's something you're doing wrong or if it's just the way your group is doing the fight. Comparing individual abilities the way you did isn't particularly helpful; sure they have similar functions (Execute vs. Soul Reaper are both executes obviously) but they have different formulas for how much damage they do and are balanced for different resource systems.
If you normally do well, my guess is that he's just tanking more than you and getting more Vengeance/Riposte uptime than you are.