A little late to the party but I thought I'd add a testimony. I've flown both VN and VJ a number of times.
Vietjet: A standard low cost carrier. I've had zero issues with them, although they -always- leave about 20 minutes late. Their website is miserable to use, to the point where I would pay a few dollars just to not deal with it. As with any LCC, check the full price with baggage before making a decision. Also consider the fact that you can accrue DL miles if flying VN.
Vietnam Airlines: Abysmal quality for a full priced airline. Bathroom doors broken; very old looking interior ; absolutely inedible food; free liquor (on international?). Bright and annoying TV screens overhead. One free bag. Flights are often less crowded.
I wouldn't pay much at all extra for Vietnam Airlines. At $30, it's a very close call, but I'd probably go with vietjet unless VN had preferable flight times.
Between HKG and SGN the best bet by far is to book CX by avios.