I think one shouldn't bother too much about weight charts, or weight numbers in general.
As long as you're eating a healthy vegan diet and feel good, there should be nothing wrong about having a few pounds "too much" or "too few"!
I agree with Michael, the standard of western society is too fat. That, and people who are overweight try (maybe unconsciously) to make slim persons seem the unhealthy ones, to feel less guilty or ugly.
I've overheard a chat between two elder women in the bus, they were both obese, as was the husband, and one told the other how sick her husband was. She responded "what, really, he looks so strong and healthy!" To me, the man looked completely UNhealthy, he was overweight and had a very red face and short breath. But to the older generation fat means enough to eat means healthy :/
Okay, that was a bit off topic
Another point is that veganism is still widely unknown and most people don't even understand what it's about. To them, it's only a fashion. Many put it in the same category as anorexia.
I think you're far from being underweight. Stay healthy, stay vegan, stay fit, and you'll prove them who's right and who's wrong
love and peace,