high humidity can be a problem<br />
mold can eventually thrive on the package and glues and cardboard center.<br />
the mold won't directly "feed" on the vinyl but will still destroy it
Heat can be a problem, but if it's less than 105F I wouldn't worry about it.
eventually, but not on a short time scale
Not sure, all my vinyl are in their covers and in cases, seem to be OK.