I have an American bf and I'm dealing with the same problem like your gf, just a little bit different is I'm living in Vn. My dad didnt say any words and walked away in the day I took my bf home to introduce him to my family. I didnt know what to say to him when he asked why my dad treated him with no respect. We are still together, even if I had said sorry for what happened but he kept using it to nag me everytime we argue about something. It makes me really tired.
Well, I came to this topic a little late and I want to tell my story. This happened more than a decade ago. I visited my friend who lived by his uncle's. After a while, I made acquaintanced with one of his uncle's girl. The girl was now 18 and I was 36 by the time. When the father knew this relationship he turned mad and banned me. Well, his daughter and I then went on meeting in coffee shops. The father is only ten years older than I. For a long while he always tried to deny me and even many times when he came home and and had to steal myself out, as he discovered my presence, he chased after me. Time and durability solved. I officially married the girl five years later.
My advice to the case of the topic is 'just to go on' and the father will accept bit by bit until he accepts the whole. To me, love is not something bad that we must hide. Be proud of it. to Waguz: you love the daughter and you have to bear all what good and bad around her, okay. Her father is her parent, no one can deny. His characters are his feelings, well, consider that okayable. Good luck to you!
Unfortunately, for most of the cases, it is true. But I'm not speaking for all men though. It also depends on their personalities and how much love they have for their children. Another problem with Viet men when they go back to VN is girls and temptations. Some of the girls are all about the money and making men happy. Again, this is just my general observation.