Custody and visa

Hello, sorry if this isn't a good place to ask this, but google brought me here.

My girlfriend is from Vietnam and i am coming to meet her in less then a week.

Thinking ahead to what will be the next step has been confusing. She has a 5 yo daughter and if we want them to come here to the USA, will she need a written\notarized letter from the father? In her divorce papers she gets sole custody but i am getting mixed advice on if the divorce papers will be enough to get a fiancee visa and a visa for her daughter. Anyone have experience with this and can give me a for sure answer? I am worried because he will never give permission if we need, he is very bitter and full of hate. I also cant move to Vietnam to be together as i also have children and cant move with them. My ex is an active mother though and the ex father hasn't been in the child's life at all.

Hopefully someone can have some advice and maybe even good news!

Thanks either way.