Not a problem. I wouldn't stress one bit, even if you book the SGN-HKG via BA and the rest via CX. Even if the flights are not linked, CX will generally take care of you in the (somewhat unlikely) chance of a misconnect. I will say one thing: you should definitely get the CX staff at SGN to print out your HKG-SFO boarding passes, as well as notify them of your upcoming reservation. CX is
excellent at having staff at the jetbridge for people with tight connections (note: 1 hour is NOT tight at HKG...I'm talking 20-40 minutes, usually due to delayed flights). But my point is, in case your SGN-HKG flight gets delayed 30-40 minutes, if you notify the SGN people in advance so they can notify the HKG staff, there might be people waiting for you on the jetbridge even if the flights aren't linked.
I'm not 100% sure if your question is about linking the tickets / getting them booked / etc, or more aligned with your title aka is 100 minutes enough time. Fyi 100 minutes = 1:20 so I think you've got a typo somewhere. I'm going to write the below assuming you've got a 1:20 minute connection (which is also pleeentttyy of time), not 1:40 (which will give you time for a shower and proper meal in the lounge, and then some).
As far as connecting airports go, HKG is just about as easy as it can get for major international airports. Especially CX to CX. There is just 1 big terminal ("Terminal 2", if you see this term referenced places, is actually just a different check-in counter area and refers to landside only. Airside is all the same big terminal). Extremely well signed. Etc.
A secondary transit security checkpoint is required to pass from arrivals to departures via transit, but that's it. Essentially transit means going from downstairs (arrivals) to upstairs (departures), all done airside without clearing immigration and seeing your bags. Your bags can interline all the way through. Assuming your connection is 1:20 (not 1:40), you still could be ~45 minutes late from SGN and make your flight. If something screwy does happen with your bags at SGN, you can go talk to one of the two large transit desks (CX has large counters open all day) which are fairly strategically located between the gates.
Here are a few links you can follow to learn a bit more about transiting HKG. I have written some lengthy posts on the topic....definitely want folks to know how easy it is to transit.
Link 1:
Which Flight Should I Take?. Post 8.
Link 2:
CX Connection Times Post 4.