Not surprising. And she wasn't denied because she didn't know your father's middle name. She was denied because she couldn't prove to the officer that she has strong enough ties to her country and that she would return to Vietnam after her visit with you.
If she tries again they are look at it like " what has changed to make us change our minds from the first time she tried"? Many foreigners treat applying for a tourist visa like a lottery by applying over and over.
It is good she has a home but what other reasons would be for her return to Vietnam? She could sell/rent her home and come live with you.
Also, and this is probably going to surprise you, but if she was coming to help you with your newborn with babysitting and stuff, that is considered work. It doesn't matter if she would have gotten paid or not. We have seen many denial because parents/relatives want to come help with child(ren).
Basically, if she wants to try again, she will have to have better evidence that she is only coming to the US for a short visit then will go back to Vietnam. And that she will not try to use the visitor visa to come and live in the US later.
