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After several months of my 90 vert turning over but not firing (occasionally), and having replaced alternator, battery, fuel pump and filter, distributor and cleaning all connections I may have finally resolved the issue. The mechanic had the car for a total of about 6 weeks and it would throw no codes, then throw intermittent codes. I searched the forums and as a last resort, bought a bypass for the VATS and so far, that seems to have done it. ($29 part and about $1200 in mechanics bills....)
Thanks to all that responded to some of my earlier posts and I really value this forum as a resource.
Not sure about the '90, so apologies if it's not as obvious, but on the '94-'96 it's incredibly obvious when your no-start condition is due to VATS. The dashboard tells you straight-up the moment you try. That's what that "SECURITY" light is about. In addition, if you're a mechanic with an ALDL scan tool, you'll find if you try to connect while VATS is locking you out that it won't connect properly either.
Kind of surprised that that wouldn't have been noticed by a decent mechanic, but glad you appear to have resolved the issue. Hope it stays resolved for you.