What is the process for a British passport holder to apply for a Non-Immigrant B visa in Ho Chi Minh while on a METV?

Hi all. British passport holder. Currently on METV, but planning to go to Ho Chi Minh to apply for Non Immigrant B.

Have emailed the embassy but no response. Does anybody have recent experience of applying for Non Imm B there. Did they ask for police clearance? It says optional on their website. If so, did they need police clearance from Thailand and home country?

Also confused about whether I need to cancel my METV first before I get a Non Imm B visa. It will still be valid for a month when I apply for Non Imm B.

Thanks in advance.