According to a bilateral agreement between the Vietnamese and Thai governments, Vietnamese passport holders can stay in Thailand for 30 days without visas if they enter by air and 15 days if they enter by land.
But under new regulations issued by the Thai Immigration Bureau, if Vietnamese citizens who frequently enter Thailand fail to prove that they're visiting Thailand for travel purposes, they will have to apply for visas.
For Vietnamese citizens who successfully prove that they're entering Thailand for travel purposes, Thai authorities will decide the length of their stay based on their travel itinerary, instead of offering them a blanket 30 day exemption.
With the new regulations, Thai authorities recommended that foreign tourists prepare papers such as return air tickets and room booking confirmations just in case Thai customs officers required them to show.