The current advice seems to be that if you are coming into Vietnam, obtain an eVisa online. These are usually available for 1 month. Then should you wish to extend, you'll need to leave the
Country and obtain a further Visa for 1 month.
If you are based in Nha Trang, you need just 1 day to complete this process:
Take the night Bus to Saigon (around $10 each way)
Arrive in Saigon early and transfer to Cambodian Border and get a new Visa (Package around $160)
Return and spend the afternoon in Saigon
Get the Night Bus back to Nha Trang
As circumstances change and 3 month tourist + business Visas are again easily available,