I was adopted from Vietnam. I never knew my real parents. I was left on the doorstep of the orphanage at birth. I have really light brown skin. I wonder if I'm half something else along with Vietnamese? Do you know your nationality?
Technically, your nationality is your citizenship. You can be any genetic background and still be a U.K. or American citizen.
But all people are a mix of every race, if they go far enough back. Still, I know I have part Cherokee, part Aztec, Spanish, English, and Italian relatives in the past five generations.
I think what you mean is ethnicity. You have the feeling and perhaps some evidence that you are not 100% Vietnamese. There is an easy and affordable way to find out. There are a number of companies offering genetic analysis. I think one of the more reliable and responsible ones is 23 and me. If you go this route, it would be wise to get the entire package -- that is, blood line and health information. The test will tell you what your genetic history is and what you need to be aware of in terms of potential health issues.
I had mine done and it was nice to see that the family story as told was pretty damned accurate.
Up til I took a DNA test a couple of years ago I thought I was Apache, Cherokee, French & English. I always suspected a bit of African American as well. After the test, well, no African American, no Native American, yes to the French and English, and 1/3 Irish thrown in just for surprise. I'm still tracking down where the Irish came from.
I'm American - ancestry is pure French. This is what my geneticist had to say on mail order genetic testing: "The ancestry testing is fun. There are some minor differences across companies in the percentage of various ethnic backgrounds, but it’s interesting to see the reports.".
One of my sisters did a mail order test that came back we were from Great Britain. My first thought was "During what time period?". So take any genetic ancestry testing with a big grain of salt. (BTW after you get Breast Cancer and it isn't the BRAC gene? You get a geneticist.).
If I were you I would have my DNA tested. The Heritage Foundation is very professional I had mine done there and I had a few surprises. The British and Irish came up on my dad's side but my mom's was a surprise. She thought she was all Italian but it turns out she was more Iberian. Also some west Asian and North African. Good luck!
I think you should concentrate on being a great person and happy in yourself. Thinking about your particular mix of DNA and that of others can lead to unfair prejudices and discrimination. Groups of people have been migrating and reproducing with other groups for millennia. You can be curious but do not let it hamper your view going forward. Best wishes.