VATS = Vehicle Anti Theft System
I believe VATS shuts off the fuel if it does not recognize the correct resisitor value, so it will crank but not start. Check the pellet. The metal contacts could be worn level with the pellet and making intermittent contact. Do an internet search for "gm vats". There is much information. Try this one for starters. http://www.asttraining.com/Files/vats1.pdf
You can check the neutral saftey switch by moving the gear selector while the key is in the crank position. If you find a spot where it will crank, then the switch is bad or out of adjustment.
Check the cables. Not only for tightness but corrosion inside the cable insulation.