Well Buddha used Bhagwat Geeta and Vedic scripts to justify his positions. Technically Buddhism is a different religion, but it uses the same Hindu logic. There were there reasons for the spread of Buddhism. 1) Ashok converted to Buddhism as you all know as Buddhism got all the resources it needed to prosper as a new religion. 2) Chinese travelers that came to India took Buddhism with them back to Tibet, China and beyond. 3) The third reinforces Buddhism outside Indian subcontinent and was probably most powerful of them all -- When Muslims attacked India the Buddhist place of learning (or the Takshila University) was destroyed and they killed or and converted all Buddhists into Muslims (as you can see in modern day Pakistan and Afghanistan). See http://en.wikipedia.org/wiki/Takshashila_University and http://en.wikipedia.org/wiki/Education_in_India for more Info