If you have no checked bags and can get a mobile BP for the VietJet flight (apparently possible according to their website) there is a reasonable but not great chance of making it.
The chance increases if you have an HKID or other means of using the e-gates in case the transit security has long queues.
Depending on the prices, I might just book both the 2015 and midnight(?) VietJet flights, unless I was willing to overnight in HK or pay the fare for booking 2 hours before flying.