I haven't done the eVisa for Vietnam, but I had a lot of issues with the eVisa site for New Zealand (NZeTA). It was recommended that I try it during what would be daytime in New Zealand, and it worked much better. Don't know if that will work in this case.
Keep trying, at different times of day. I tried about 4 or 5 times over a span of 3 days, it was a pain, but it finally worked for me early in the morning.