You can request it, but the airline is going to want to change your ticket to match the new routing...which is likely to involve you shelling out a considerable amount of money.

If you don't have checked bags, you could try leaving the aircraft at SGN, but cabin crew are likely ask to check your boarding pass as you leave. That's not so much to protect revenue, more to avoid passengers mistakenly abandoning the flight, and to avoid landing passengers who carry documents that haven't been checked for Vietnam entry.

You're probably going to have to accept that PNH is your destination, and set any inconvenience against the money you're saving on a cheap ticket.