I am a UK citizen in Vietnam looking to get to Thailand on the STV. A couple of months ago the embassy in Hanoi said that they could get me this visa without me having to be in my home country. Today I enquired with the HCMC embassy and they told me I must be in my home country. I asked if I could go in with a regular Tourist visa and then switch visa when I am in Thailand (as I have read many time this is possible) but HCMC just told me no I can't!
TLDR : Answer Summary
A UK citizen in Vietnam cannot apply for the Special Tourist Visa (STV) from abroad; it must be applied for in their home country or where they are a permanent resident. Attempting to switch from a regular Tourist visa to an STV in Thailand is also not permitted. Options include applying for a tourist visa with the possibility of extending it once in Thailand, or utilizing visa-exempt entry for a limited stay.