Interesting question. I've never been in such a situation myself but can understand the concerns.

I would turn it around a bit and look at it another way. I think a lot depends on how confident your Mother-in-law is as a traveller.

For example, if I reversed the route and applied it to myself going to Vietnam, I would not be uncomfortable with making the journey. But I am a reasonably comfortable traveller who is not too bothered about language differences. At the same time though I do speak English and every airport in the world has signs in English to help me know where to go. A list of the typical signs she will encounter with translations into Vietnamese might help her.