I agree that being aluminum the risk of worrisome corrosion should be nil as long as the valve was on the cylinder so rain and dead mice are not in them. If the valve was closed and better still there was any residual pressure so much the better.
From a hydro performance being empty is best case since the cylinder is not under any stress. Again, not a concern. There are a gazillion such cylinders that have been in service, enduring pressure cycles from that same era and hydro failures for elastic performance failure are rare. Again, dormancy is your friend.
However I do see a possible catch here..... being 1990 cylinders they are 21 years old. There are shops that are blackballing cylinders over 20 years old and simply refusing to fill them. This may apply to any brand and alloy, a simple 20 year cut-off. Be sure you understand the local shops sentiment about this and if it's favorable hope they don't change their mind.
If in otherwise good shape figure $45 for VIP/Hydro to recommission each and go from there. Given age and risk getting them for $30 each would be nice. At $50 each you still have a decent value if it all works out (highly likely). A new cylinder is in the $150.+ range so that is what you need to come up favorable to.
The age, hence potential blackball are my biggest concern. It's totally unfounded moronic hogwash but that may be the reality you face.
Pete
