Nice description of "suction feed" tests carried out during engine certification in this article:
Rolls considers keeping 747 for future engine trials: AINonline
For the OP, the reason engines flame out under suction feed is vapourisation in the feed lines. As has been stated, after prolonged flight there will be a certain amount of deaeration allowing a relight at lower ambient pressure.
Most vent systems are designed to put a slight positive pressure in the tanks but it's only small. One of the requirements of a tank system is it must not fail with a 3.5psid pressure (CS25).
Just to give an idea of what can go wrong. Embraer had to modify the naca fuel vent inlet on the 190 after it was found that at certain climb speed and weight combinations the vent works the wrong way round and creates a syphon effect which combined with a flap valve failure in the vent tank baffle can lead to venting a lot of fuel very quickly!