While it is true that disease did most of the dirty work, what jackass thinks that being "vegetarian" has anything to do with it....New World inhabitants were largely vegetarian with essentially no domesticated food animals, who had no genetic resistance to the devastating Old World microbes."
The native americans tribes ranged from hunter gatherers to hortaculturalists to full on agriculture. They were in no way vegetarian. They had domesticated animals. They had population density. They just didn't have small pox.
When Columbus landed and brought "civilization" to the Natives, the New World had 2-3 of the worlds LARGEST cities and the worlds LARGEST empire.