I hated my domestic to intl transfer at SGN. There is virtually no signage telling you where to go anywhere in the airport. But you will need to collect your luggage at the domestic terminal and recheck it at international. The walk between terminals is not really all that far, but if your mother has mobility issues I like the idea of trying to arrange for a wheelchair.

My transfer was from Da Nang. The airport there is very small. Check in is not allowed until rather close to flight time if I recall correctly. So no need to arrive too early for your domestic flight. I would guess Hue is similar. Actually Hue airport is even smaller.

Once I got to the international terminal I was again struck by the absolute lack of any signage. It took me a while to get checked in and then find the VN lounge. I seem to recall that the security set up there is also rather strange but I can't recall the details ATM. I believe there was a long line at passport control and then you needed to go get in another long line for security check.

But I think two hours ought to be plenty of time to connect.

I swore I would never use that airport ever again.

Good luck!