I'd be a little concerned about your aftermarket alarm installer. If he says he installed a resistor to match, then the VATS system is bypassed - that's the only reason to add a resistor. Otherwise the system reads the resistor pellet in your ignition key.
It's not unusual for the system to malfunction because of simple things like grime on the key resistor pellet. Try using a pencil eraser and then some rubbing alcohol to clean the contacts on the key (or try your spare key). Also, grime can affect the contacts inside the ignition cylinder. A little contact cleaner spray (Radio Shack) and a pipe cleaner will usually take care of that problem. All this is pointless if there has been another resistor installed in the system. But that is usually only done for remote starters - it's just not necessary for an alarm system.