If the starter isn't spinning, it's not a VATS problem--VATS kills fuel, not the starter.
You need to take a look at the wiring diagram for the starter circuit, and rig up a test light (or voltmeter) with a really long cord to carry with you so you can troubleshoot the problem the next time it happens. It could be the ignition switch, the clutch switch, the starter relay, the starter solenoid, the starter, or a battery cable.
The first test I'd do next time it happens is to clip onto the terminal of the purple wire at the starter and see if you are seeing 12V there when you turn the key to start. If you are seeing 12V there, then you have a starter/solenoid/batterycable problem. If you don't see 12V, then the next thing I'd check is to pull out the starter relay and check whether you see 12V on the coil hot terminal (the one that runs to the ignition switch, see wiring digram to figure out which that is). If you do see 12V there, then the problem is the relay or the wire going between the relay and the solenoid. Etc...