Did Buddhism oppose Vedic religion?
In his Great Minds of the Eastern tradition lecture series Grant Hardy identifies 6 orthodox schools of Indian philosophy (Samkhya, Yoga, Nyaya, Vaisheshik, Mimāṃsā, Vedanta). He also identified 3 heterodox schools 


Cārvāka (materialist) 
Jainism  
Buddhism


The heterodox schools are those who reject the authority of the Vedas. So based on this then Buddhism was opposed to the Vedic religion.Interestingly the Buddha came to be seen as an avatar of the god Vishnu in some forms Hinduism. I guess that it's better to have the Buddha inside the tent chanting out then outside the tent chanting in (or words to that effect).