Really, how do vegans exist? are there any nutrients that require the eating of animal parts?

Vegans: They do very well. There are many plant options that will make up for eating meats or as you put it nimal parts.

Supplement: Amateur vegans / vegetarians often suffer nervous system damage, sometimes permanently, if they fail to supplement with vitamin b12. They may also run low on calcium, iron, zinc, vitamin d, usable vitamin a, and iodine. If they know what they're doing, they can supplement, and this seems to work fine. The health benefits are mostly subjective but they seem happy with the lifestyle.