1st generation just means that your parents were born in Vietnam. It sounds like you have some great reasons for joining. Just put those into your essay and you should do fine. I think the most important thing that they look for is honesty and sincerity. They don't want something like "I want to join the Navy because I really want to put a .50 cal slug in Haji's head from the door of a 60", but you should still be honest. If that was really your reason, then you could say "I want to take an active role in fighting global terrorism."
In your case I think it will help, not hurt that your dad and uncle were in the Vietnamese Navy. It will show them that you have a true dedication to forwarding the cause of freedom.
In your case I think it will help, not hurt that your dad and uncle were in the Vietnamese Navy. It will show them that you have a true dedication to forwarding the cause of freedom.