This is based on the assumption that:
1. The Sentinel/Mara gets the jump on me first and can open up fully with his abilities shortly after engaging me. On my Shadow Tank this rarely happens except in Huttball where you are much more likely to be singled out away from your team. My Shadow is usually stealthed trying to achieve objectives on ALL maps and in the case I see a Mara/Sent approaching I stealth and get the jump on them first. On my Vig specced Knight it doesn't matter. I've learned to play him so well that I've burned Maras down in seconds. Doesn't happen all the time but it happens enough. On my level 35 Smuggler I've burned Maras down by keeping them at range. On my smuggler it is much harder but it can be done. There is always a tactic to take down any class in this game if you've observed or played them all.
2. I had another but I forgot but the above more than makes up for it. I have a Mara and I'm learning more and more about the class and how to play him but it is hard work but also rewarding. I think my Mara was the first class I broke 200k damage with. Since then I've done it with my Vig specced Knight and my Balance assasin but the Mara really helped me to appreciate the class and that all the whining I was doing was nothing. It's the skill of the player and some players can play them very well. I can tell when I'm up against a noob Mara/Sent in pvp because of how quickly I've beaten them.
Considering equal gear, any sent that loses to a vigilance guardian is just plain bad.