Can I apply for a Non-O visa for marriage from Vietnam, and what are the financial requirements?

You can apply in Hanoi, there are many agents there, too.

Many local Thai Immigration Offices inside Thailand will not consider or accept newlywed Non–O Visa based on Marriage to Thai Spouse or Extension of Stay based on Marriage to Thai Spouse until Married for a couple months or more

As for Thai Embassies or Thai Consulates outside Thailand for a Non–O E–Visa based on Marriage to a Thai Spouse, would definitely try to directly contact and confirm with the specific Thai Embassy/Thai Consulate beforehand whether they have any specific requirements for minimum time to be Married or not and accept newlywed Non–O E–Visa Applications or need to Married for a couple months or more

You can apply in Vietnam for the initial 90 day Non-O based on marriage by showing 400k baht in your home bank. When you get to Thailand you'll need to open a bank account and put 400k baht in it, when it's been in the account for 2 months you can apply for the extension of stay based on marriage. You can't use 12 months of 40k transfers from overseas for that first extension, and depending on your immigration office policy you might never be able to use them. An increasing number of immigration offices will only allow that 40k baht if it is from income earned inside Thailand so ask them what their policy is when you go to pick up their list of requirements prior to applying for that 12 months extension of stay, every immigration office can be slightly different so be sure to get the list from the office you will actually use.