Should I apply for a Non-O visa in Vietnam or enter Thailand with a tourist visa and convert it later?

If you need to open a Thai Bank account, you will need to get the Non-O eVisa from outside Thailand. Enter on the 90day Non-O. Then get your condo lease ASAP. You'll need the condo lease (if you don't own a condo) in order to open the Thai bank account. Go to immigration (with the condo lease) to get the resident letter from them which is required to open the bank account. The letter from immigration is specific, so you tell immigration which bank you want to open the account, and they'll address the letter to that bank. Go to the bank with passport, Non-O visa, condo contract (or your house book if are an owner), immigration resident letter, and get the account opened. Immediately transfer in the 800k from your home country bank account (it must show as a foreign transfer). The funds must be in the account at least 2 months before you apply for the 1 year Extension of Stay, so you need to get all of this done within the first 2-3 weeks of entering on the initial 90day Non-O. When you arrive, you may want to double check with the bank as to what (if anything) else they may require to open the account. Good luck.

If you already have a Thai bank account you can come in visa exempt, but if you don't you will have difficulty opening one on that so get the Non-O before you come

You've got your wires crossed a bit

Any way you look at it you need to get the 90 day Non-O visa FIRST before you can do the year extension at the immigration office after it runs down.

You can't get a Non-O visa for a year from the thai consulate in vietnam, you get a 90 day Non-O visa from them (same as you'd get inside the country)

OR

You come in free stamp, visa exempt, get the 800K baht in a thai bank account in your name and then apply for the 90 day Non-O at the immigration office where you stay.