evergrn

If you'll really only have carry-ons and you can confirm with Vietjet that you'll be able to do online check-in ahead of time, then I think your chances of making the 20:15 flight is excellent. The ability to do online check-in ahead of time for Vietjet is crucial, because 1).you'll have BP to present at transit security in HK; 2).every airline has a check-in deadline (60min for example).

Even then, even as I'm saying you have excellent chance at making that 20.15pm flight, I still agree with others. There's no way I'm taking that chance. I think I would want at least 2hrs from scheduled arrival to departure. The 22:30pm CX flight is perfect. If that flight's too expensive or you don't like the arrival time in SGN, well then I would just suck it up and plan on spending the night near HKIA then continue on the next day.

s0ssos

CX charges an arm and a leg for Vietnam flights.

cxfan1960

They do if you buy the flights separately, but unlikely if that is an add-on.

s0ssos

Can you actually add-on for free? I don't know CX's rules, but for most airlines you cannot change a ticket for cheap, and in the US the fee is like $250 making most changes moot. You just throw away the ticket and buy another.
The OP already has a ticket to HKG.

cxfan1960

I see. It depends on the CX ticket. If bought through a TA, there may be additional fee too. My add-ons to TPE and PEK from SFO are usually free or almost free.

Clipper801

The key is to search SFO-SGN on CX web site.
Buying 2 separate tickets with such short connecting time is dangerous. I shall not do it. If the CX flight is late causing a missed connection, you are on your own. CX is not responsible for the missed onward flight. Basic Travel 101.