I would mix it dry and store it dry. The only exception would be for someone with asthma, COPD or some other respiratory issue where they probably ought to mix it damp and store it damp to preclude the possibility of there being any dust from it floating around to be inhaled.

There's no reason not to use your Metro Mix. I use leftover bags all the time, with the caveat being that I stored them in a manner that the mix stayed dry. The only exception might be if I opened the bag and it had some sort of odd smell, then I might dump it on the compost post figuring the odd smell might indicate fungi or something had been growing in it.

Perlite has largely replaced vermiculite because of an unfortunate incident where the vermiculite from one particular mine was found to be contaminated with asbestos. That mine was closed down and there is no reason to fear that vermiculite sold nowadays would have the same issue, so there's no reason not to use it, really. However, ever since that happened, some people just want nothing to do with vermiculite. Often, it just depends on what you can find. The stores where I shop are more likely to have bagged perlite in larger sized bags than vermiculite, which pretty much makes up a person's mind for them. You buy and use whatever you happen to find.

The asbestos scare aside, vermiculite does not release any minerals and has a very unfortunate habit of collapsing once it becomes fully saturated with water. And as that happens quite rapidly in a container situation, you lose any porosity and drainage enhancement the vermiculite might have added and then the potting mix itself starts to compact and begins to drains poorly. This is why it is not included in any better quality potting mixes - perlite, pumice or turface hold their shape indefinitely and continue to provide aeration and fast drainage, which is why they are preferred.

If potting mix stays dry, it can be held indefinitely without concern or deterioration.

Thank you Dawn and Gardengal48. I appreciate the info, as I've never mixed my own potting mix before. I don't generally grow much in containers except a few houseplants.