Restart when stalled while moving

If the car is moving, you won't be able to engage any gear without clutching it. Or perhaps you could engage those over 2nd, depending in your gearbox. Nevertheless, you could clutch it, engage a correct gear depending on the moving speed, and release the clutch not too fast, to avoid any bumping reaction. When the car stalls in a situation like you described, there is no need to turn off the key: just clutch, shift as appropriated, and release clutch.

Sometimes, when I'm too low on gas, I use to get into some good speed, then turn off the key, shift to neutral (or engaged if doing down a hill) and let the car move by itself...when the speed gets slower, then I would turn the key without starter, clutch->engage->release to resurrect the thing :) In fact, I wouldn't use the starter at all, if I'm down a hill...just let it run down a bit and crank it by itself, in a same procedure. It is a routine of mine, don't know if good or bad, just got very used to it since once I got a bad starter for weeks.

EDIT: This procedure is not good for those cars provided with power steering, since it could make it too hard to maneuver.

The van was likely not fully stalled, just shuddering at very low revs in 6th gear, as 6th gear will have been too high for your road speed. The van likely only stalled when you depressed the clutch, as the engine would still be rotating if you were still moving & the van was in a gear.

Unless the van is fitted with some sort of reset mechanism/switch.. There was no need to switch off the ignition really. Because as long as you were moving above say about 6mph, you could have just selected the correct gear and released the clutch whilst giving a little throttle. This would have spun the engine back into life again via the transmission.