Well, i just applied and obtained a Non B 90 days visa with documents provided by the BOI in Ho Chi Minh City Vietnam few weeks ago.
My choice for the consulate of HCMC was because I also working in Vietnam, based in HCMC, so convenient for me.
There was no much waiting list for the appointment. I choose a date 2 weeks later when I made the appointment on their website at it convenient for me but i had the possibilities to get it earlier.
On the of the visa application, all went smooth. There was only 1 document regarding the company taxes missing. They allowed to ask my company in Thailand to send me this document by email and get it print in the shop nearby and come back to the consulate. Then all done in 45 minutes.
The payment (80 USD) must be done in USD only. Be careful as they are strict on the quality of the USD notes. If you don't have USD, they indicate you a place nearby to exchange. For me, I had USD already with me.
I applied and the Friday morning and collected my passport back on the Tuesday afternoon.
FYI, in my case, my company is relocating me from Vietnam to Thailand.