I'm planning to apply for the evisa in Vietnam through Thai soft power (Muay Thai).
I'm currently in Thailand. Do I have to leave thailand first before I can submit my online application on [members only]/
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TLDR : Answer Summary
To apply for the eVisa while currently in Thailand, you must first leave Thailand and go to Vietnam. The application requires you to provide your current location and the embassy will need your address in Vietnam. Attempting to apply for the eVisa from within Thailand may lead to rejection and potential loss of the visa fee.
Suggest you READ THE POSTS... you will know the answer. Especially read the posts about getting the DTV visa with Muay Thai training. There are requirements which must be met.
Arya *****
Sunday I’ll go to Vietnam for dtv also learn Muay Thai
That's one part of it, but some others mention including a photo of their passport, showing the entry stamp to the country, where the application is being made.
If, for the sake of argument, one booked a hotel in Hanoi and used that booking as evidence, whilst they're actually applying from Thailand, it might work. What if they ask for more evidence, such as a photo of the entry stamp to Vietnam?
I'm not saying it's likely, but it's certainly possible. With the e-visa process, if your application is rejected, you lose the fee.
In that case, I'm not sure how you can truly verify your current location.
Some folks in this group believe that the embassies/consulates can check your immigration status in Thailand. If that's the case, at least they can verify that you're out of the country...
what about your address and visa in Vietnam… what if they ask you to submit the proof of visa stamp from passport… You can try that through VPN and share the experience
I don’t think they ask that, they certainly didn’t with me, they asked for my address, if you’re staying in a hotel in Vietnam, that is your address, why would they ask for a visa stamp? 
no I did not use a VPN, I live in Canada and I applied in Canada, but I could’ve been in Vietnam or Cambodia or Taiwan or China, as long as I have an address in Canada I could’ve applied anywhere in the world