Business visa can only be approved under some conditions:
1. Have sponsor company ( NO AGENCY)
2. Stamp visa at Viet Nam embassies/ consulates before entry Viet Nam ( No visa on arrival at Viet Nam airport and ground border)
Airport letters are only issued if the country you're flying from doesn't have any embassy/consulate.
For those who already have visa letter, visa stickers, 5 years exemption or TRC you might not be able to entry IF:
1. Travel history includes European countries + other serious affected areas.
2. Did not pass the medical check at your entry port ( you'll either put under quarantine or sent back to where you came from)
For visa extension inside Viet Nam without leaving.
1. Price will be most likely be higher that usual
2. You will have to fill in the medical declaration. ( https://tokhaiyte.vn/ )