Should a qualifying lap in which all four tyres are off the race track be ignored?
No, it should not be ignored. It should be judged. Depending on where the car leaves the track time will be lost or gained. Standard practise in the past has been to erase the lap time of the lap on which the offense/mistake occured. Until the FIA tightened up the rule on off track excursions it was applied to specific corners on specific circuits - e.g. chicanes at Magny Cours and Monza, etc. The issue of exiting a corner and running wide was also not an issue in previous decades as it has been the replacement of gravel tracks with tarmac tun-off's that has created the problem.
With regard to Vettel running wide in qualifying, he used all of the circuit and more in order to carry more speed through the corner and have a faster entry onto the next "straight". That is, therefore, gaining a time advantage probably worth around 10th of a second. With this season being so close that might than have been the difference between him and the next guy on the grid. Intentional or otherwise is irrelevant since no driver is going to say "I meant to go that fast in the corner and knew I could run out wide and get away with it," is he?