from the description you've given this would indicate a fault with either the crankshaft position sensor or a base engine fault where there is to much end float on the crankshaft.
The engine position used by the engine management is taken from the crank position sensor, this uses the flywheel to detect the position of the crankshaft. If this signal is missing the engine won't start. When you press the clutch this pushes the release bearing of the clutch into the clutch/flywheel assembly, if there is excess end float on the crank it may be the flywheel has moved far enough away from the crank sensor to lose the flywheel position but when you press the clutch this pushes the crank and flywheel back into a position where the flywheel position can be detected again so the engine starts.
Once the engine is running the voltage from the sensor is higher so the engine management can detect it better so the engine continues to run.
There is a possibility that the starter motor is taking too much power when cranking the engine, this can shut the engine management down but if you press the clutch the reduced drag from the transmission may be enough to reduce the amount of power taken by the starter to a point where the engine management remains active so the engine starts.
Start with checking these points, also if you have just got the van make sure there isn't some kind of aftermarket cut off fitted to the clutch pedal that won't allow the engine to start unless the clutch is pressed ( disregard this if you have had the van for a while and the problem has just occurred).
Let me know if I can advise further and I'll reply as soon as I'm able to.