United Kingdom
45 free days, and then the arithmetic gets awkward
A British passport is on Vietnam's unilateral exemption list. Under Resolution 44/NQ-CP, signed on 7 March 2025, citizens of the UK and 11 other countries get 45 days from the date of entry with no pre-approval, no fee and no stated limit on the purpose of the visit. The policy runs from 15 March 2025 to 14 March 2028.
Two details trip people up. First, British National (Overseas) passports are not covered; a BNO holder needs a visa like anyone else. Second, the 45 days cannot be extended inside Vietnam. When day 45 arrives you leave, and the run is the only way back.
Coming back on the exemption again is allowed. The old 30-day cooling-off rule belonged to the 15-day scheme and is gone, so there is nothing on paper stopping a second exemption entry. The risk is not legal, it is visual. Two exemption stamps and a couple of border stamps inside a year reads like living here, and that is the pattern officers ask about. Most long-stay Brits switch to the 90-day e-visa after the second free entry and stop thinking about it.
The e-visa is open to British passports too, at the same 25 USD for single entry and 50 USD for multiple that everyone else pays. Ninety days at a time means two runs a year instead of six, which is why the paid option usually wins for anyone staying a season. If you are a two-week tourist who fancies a weekend in Phnom Penh, take the free stamp and keep your 25 dollars.