Keep in mind that any interaction with them, whether a warning, or even refusal, is in the database, and immigration's computer system is getting pretty advanced very fast, it's better to just wait in Vietnam until you get the DTV. That already does what they would warn you about, or refuse you, so it looks good to immigration.
the basic requirement is that the person must have the equivalent of at least bt500,000 in a bank account in her name only. Without that nothing else matters for the DTV. If she has that then there are lots of options, and easy to research.
You must have the FET from the bank showing that the money to buy it came from overseas, or that you worked in Thailand, and paid Thai income taxes on all the money you used to buy it.
If you did not check the "for long term visa" box they will not be listed as international transfers on your Thai bank statement that immigration accepts for verification, unless you are from one of the few countries whose embassy still issues income verification letters. Most don't
as it stands right now, and you already have the required money in the bank, your best option for the next five years is the soft power category of the new Destination Thailand Visa. It's the new five year multi-entry visa that stamps you in for 180 days on each entry, and you can extend it once each year. It is so new that rules and regulations to govern abuses they find haven't been put in place. If you get it now you would be grandfathered in for any changes for the next five years. Basically you just need at least bt500,000 in your bank, and you have that.