Although there are many reasons that a vehicle may fail to start, a problem common to VW camper vans has to do with the Bosch brand starters that VW uses. If your van fails to start when it's hot outside, this is likely your problem. Other symptoms include turning the key and having nothing happen. In this scenario, the starter won't even begin to crank the engine. Because they use an electromagnetic solenoid to activate the starter mechanism, Bosch starters are prone to failure when the weather is hot. Electromagnetic forces are weakened, and sometimes reduced to the point that the solenoid can't activate the starter.
Turn on your headlights to see if they are as bright as they should be. This helps determine the amount of charge your battery has. If available, use a voltmeter to determine your charge.
Charge your battery if it is low or dead. Disconnect the solenoid wire that comes out of the ignition switch. Check the voltage with a volt meter. It should read 12 volts. If there isn't 12 volts coming from the solenoid wire, you likely have a defective ignition switch that will need to be repaired.
Check if the starter's motor is the problem by connecting the battery directly to the starter without engaging the solenoid. Put your van in neutral and engage the parking brake. Use an insulated pair of pliers or a screwdriver and lay it across the two post electrical connectors on the back of the solenoid. Be prepared for sparks.
Listen for the starter's motor to spin (it won't crank the engine). If it doesn't sound smooth and normal, or if it doesn't spin at all, you have determined the starter's motor is the culprit and will need to have it rebuilt.
Use a voltmeter to test if your solenoid is activating. The right-hand post electrical connector on the back of the solenoid should have a braid of wires coming from it. Spike the voltmeter's positive lead into the braid and ground the negative lead. Have someone turn the key. You should hear the engine crank and be reading 12 volts on the voltmeter. If not, your solenoid is bad.
Remove the wire from the ignition switch from the solenoid to further test your van's solenoid. Once again short the big post on the back of the solenoid (the one with all the wires) where the ignition switch was plugged in. This should cause your engine to crank.
Buy a starter relay kit and wire it into the circuit to improve starting reliability and to remedy the VW's notorious "hot start" problem. Alternatively, you could have the solenoid rebuilt as a cheaper way to remedy the issue.