On Jetstar A321s, the SOP is that ALL doors are disarmed when parked at the gate, but I've commented on a similar story to this on another forum and was informed that some airlines leave the mid-cabin exits armed on the ground.
I think that's a very bad idea because something like this incident with Vietnam Airlines could happen.
It's not just the cost of re-packing the slide and checking the door mechanisms, etc. They're also very lucky that there weren't any ground staff working in vicinity of the door that was opened because one of those escape slides could severely injure or kill anyone that they strike, considering the rate at which they inflate.
And Philip, with your question about how a passenger came to be adjacent to the door handle, well on the A321 passengers seated in 10A, 10F, 24A and 24F (window seats of the emergency exit rows) have the door operating handle of L2, R2, L3 or R3 (whichever applicable) positioned about 30-40cm in front of them. The operating handle is covered only by a plastic cover guard which is spring-loaded and if pushed away, the slide armed warning indicator light will illuminate and the flight deck will be given a warning that a cabin door is about to be opened in armed mode.
The passenger's attempt at blaming the cabin crew about not giving him a thorough enough explanation is weak. No matter WHAT kind of briefing you're given about the operation of emergency exits, however thorough, accurate or confusing, do you think you would dare touch ANYTHING on the door under normal circumstances? The idea is not to "give it a go" once the flight attendant has just told you how to open it!
Anyway, this is further proof that airlines around the world operating aircraft with mid-cabin exits within easy access of passengers should change SOPs to have ALL doors disarmed at the same time (when parked at a gate/on a bay).