It's not that the surname is not used anymore, it just wasn't common to begin with. Since I'm Chinese and Vietnamese, there are many Chinese with the same pronunciation, but not many Vietnamese.
I have family all over the world since we left Vietnam in the 70's. I'm still finding out about them. Majority of my family is in Virginia or Hawaii. There are Tiet's in Australia!
Don't know if I've helped....good luck,