7-Dehydrocholesterol has significant absorption of UV light at the lowest wavelengths of light found in sunlight (290-310 nm). Hence the need for full-spectrum lamps to emit UV-B radiation (UV-B light is defined here to be light between 290-315 NM).
In rare cases some vegans might not get enough fat or calories to produce adequate amounts of steroid hormones (which are produced from cholesterol). Two studies have shown vegans to have sex hormones on par with meat-eaters (1, 2), but one report showed vegan women to have lower levels of estrogen (3).
Vegan diets in particular tend to be low in calories for obvious reasons (e.g. a salad vs. a 1/4 pounder with cheese). To build muscle mass with vegan macros, you have to pay special attention to calorie intake. That is, you need to consume more calories than you metabolically burn during and after exercise.