You can assist your mother in law in filling out the B1B2 visa application online, but she should fill it out herself and must electronically sign it herself in Vietnam. That shouldn't be hard with Facetime, FB Messenger or whatever. After filling the application, you go to the HCM embassy/consulate website and fill out the visa application online. Then you pay the fee at the post office. After that you schedule the visa interview. On subsequent tourist trips to USA, its easier bc she could do mail-in visa application if she qualifies.
Does not need lots of money or property in Vietnam. (honestly i think it's the dich vu people trying to size up people to charge them accordingly).
But she does need reason for visiting USA and a reason for going back (like a spouse or children to return to in Vietnam).
Some people think it's better for her to have at least one plane trip outside Vietnam to a place like Thailand, Singapore, Hong Kong first before visiting the USA. I would agree because it puts her "on the grid", instead of just showing up from the mountains or the jungle.