Heroes is an lot said but as you know the republic would have lost against the empire of not for the 4 heroes stopping the sith. Would the sith empire have lost against the republic without the 4 heroes of its faction?

Yes, mostly because of Makeb, which was the real watershed for turning around the Empire's fortunes.

I think neither side would have won; as if you assume no heroes on the Imperial side, that means no heroes on the republic side either. Vitiate would have been able to successfully destroy worlds such as Belsavis, Revan would not have been freed, and Vitiate would not have had to wait to start eating worlds. He would not have to wait to be "freed" from his Voss "self", he would have found some other powerful Sith (such as Baras or Malgus) to work through. As Revan was already becoming consumed thru his captivity, he would have subsumed him completely and thus been stronger.

 

Vitiate would have eaten the galaxy. Nobody - republic nor sith - would have survived.