Nuts are not high protein. I don't know where this idea comes from. There's protein in nuts, but compared to the amount of calories per gram it's really not that high. Unless you want to gain weight nuts shouldn't be your main source of proteins.
Anyway my point is just that the protein part is important because that's the part that people need to change their diet for. Everything else people wouldn't really have any issues going vegan.
Most studies I've seen on the subject could just as well explain the longer life span of a vegan diet by the fact that vegans are more careful about what they eat and tend to be more health conscious in general. It's extremely hard to actually associate a longer lifespan with just a particular choice of diet because there are a lot more factors than just diet that impacts this.