The fact that Vivec would bow before a mortal doesn't lessen him as a person. It elevates him to a new level. Some people wouldn't recognize class and humility if it bit them in the face. With great power comes great responsibility. Any god that goes around neglecting and stomping mortals like ants isn't going to be very popular. Even the Daedra show some restraint.
As for how "Strong" they were...I guess you're looking for some kind of battle power statistic?
We never see them at their strongest. During the events of Morrowind and Tribunal it's been hundreds of years since they had access to the Heart Chamber. At the peak of their power, they could bring the dead back to life, could cast a water breathing spell on every person in Morrowind simultaneously, could duel a Daedric Prince (with some difficulty) and freeze a meteor in time and space. Those are very impressive feats by any standard - far above what single mortals could achieve in their stead.
But as for "who would win" topics I'm not sure those are applicable. Sotha Sil and Almalexia beat Mehrunes Dagon, but it took both of them and Almalexia was badly wounded. To complicate matters further, they had the home field advantage - Daedric Princes would not be as strong on Nirn as within their own realms. He didn't have access to his full strength. So if we just had to do a "who would win" topic they're weaker than the Daedric Princes, and weaker than the avatar of Akatosh who appeared in the Imperial City. I think. That's how I'd rank them anyway.