I am from the UK. Do I need a visa for 3 weeks in Vietnam?

No, UK citizens can enter Vietnam without a visa for up to 45 days, so a 3-week trip does not need an e-visa. Your passport must be valid for at least 6 months from the date you arrive and have a blank page.

You still need to do two things. First, complete the free digital arrival card at prearrival.immigration.gov.vn within 72 hours before landing. Second, keep proof of onward travel handy, because airlines may ask for it at check-in.

The 45 days count from the day you enter. If you might stay longer, an e-visa gives up to 90 days, and it is a better choice than relying on a short extension.

British National (Overseas) passports are not covered by the UK exemption and need an e-visa. See the exemption guide for details, and check xuatnhapcanh.gov.vn for the current rule before you travel.