Hi,,,I am flying to Thailand in April,,,I have a return ticket back to UK the end of may,,,after 2 weeks in Thailand I am flying to Vietnam for 1 week,then back to Thailand then flying home end of may,,,,my question is,do I need a visa for Thailand,,

Not if you have a passport eligible for via exempt entry. 

Providing you do not have something in your history of previous visits, and you are on a UK passport, or a passport that eligible for visa exempt, then your plan seems ok,

 

But your second entry seems it could be up to 5 weeks (clear dates not posted), so be aware the airline from Vietnam may require a ticket out after 30 days, so may need throw away booking.

Your airline could require a retur

3 minutes ago, Georgealbert said:

Providing you do not have something in your history of previous visits, and you are on a UK passport, or a passport that eligible for visa exempt, then your plan seems ok,

 

But your second entry seems it could be up to 5 weeks (clear dates not posted), so be aware the airline from Vietnam may require a ticket out after 30 days, so may need throw away booking.

As said above will depend on amount of time last trip in Thailand and airline policy - normally Thailand immigration is not an issue but they could be.

Quote

Foreigners entering Thailand by any means under the Visa Exemption scheme are required at the port of entry to have proof of planned travel (confirmed air, train, bus, or boat tickets) to leave Thailand within 30 days of the arrival date. Otherwise, a visa must be obtained before entering Thailand.