There does not appear to be a rule covering Vietnam so not sure where you got that from - I just therefore assumed it was because of the CX/VN JSA.
So it would seem that you are free to pick up a CX flight from SGN through to HKG if you so desire.
Star files can be found here:
http://www.hardlink.net/~markdu/OWFi...eExplorer.html
edit: It seems that there may have been a time when the CX/VN flights on the SGN-HKG route were not allowed on a xONEx fare.