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Being underweight is generally considered to be an increased risk of health problems, such as lower resistance to infections, slower wound healing, heart problems (not generally the same ones that obese people have), thyroid problems, hair loss, lack of fertility for women, bone loss / osteoporosis.

The usual guideline of BMI 19 to 25 suggests that, for your height, a weight of 97 to 127 pounds is the "healthy" range.

However, when gaining weight, include some muscle building exercise so that you gain some muscle, instead of putting it all on as fat, which can result in the "skinny fat" condition ("normal" weight, but high body fat, which is not healthy either). However, some body fat is necessary; the American Council on Exercise suggests that the minimum essential body fat for women is 10-12%, usually only approached by athletes; the amount in women with good fitness goes up to about 24%, though up to 31% is "acceptable".

A vegan diet is likely to be deficient in vitamin B12, which (in modern rich countries) only comes from animal source foods, fortified foods, and supplements. Vegan food sources of vitamin D other than fortified foods are uncommon, so if you do not get enough sun exposure on your skin, then that may be a concern. Vegan calcium sources include several types of green leafy vegetables and tofu made with calcium sulfate. Women have a higher iron requirement than men, but are less prone to eating high iron foods; eating vegetable foods with iron along with vitamin C containing foods increases the iron absorption.