This may have an incredibly obvious answer but I'm curious:

In going through the Collections interface, I find reference to various things such as packs (like the Vice Commandant's Contraband Pack). I can't purchase things from the Collections but it tells me that if I purchase them from the Cartel Market, those items will be part of my collection.

However ... for many items, I can't find them on the Cartel Market at all. As just one example, the Vice Commandant's Contraband Pack appears to be nowhere.

I do see other packs so I don't know if the Vice Commandant one has been rolled up into those or something.

Most packs are only available for a certain time and then taken off the market. Sometimes they make a brief return, such as some of the recent Hypercrates.

You can buy most items off the GTN and add them too your collection, but note that, in the case of armor sets you need to purchase and equip the whole set before it becomes available to purchase/obtain from the Cartel Market.

The part about buying things off the Cartel market is a bit misleading - most of the items are not directly purchaseable from the market - it's only after you get them from a pack or buy them from the GTN that they become 'download-able" from the market through you collection.

I also found this confusing, but don't think we can buy items through Collections. By spending a Cartel coin fee, collections lets us unlock items we already own so that copies of them can be used by any characters we make on our account. So if I go and buy an outfit that was in a Cartel Pack off the auction house, and then equip the whole outfit (all pieces) on my character, I will see that outfit show up in my Collections. Now I can spend Cartel money to allow all characters on my account the ability to freely take copies of that outfit. I paid this account unlock fee on a Cartel Pack mount I got off the auction house. Now any character I make will be able to use a copy of that mount, and I won't have to rebuy it for future characters.

I also got confused between Collections and Legacy unlocks - took me a while to realize that Collections is a tab in my inventory, not a part of my Legacy screen.

So if I go and buy an outfit that was in a Cartel Pack off the auction house, and then equip the whole outfit (all pieces) on my character,

You don't actually have to equip all the pieces on the same character (afaik). I unlocked a set, but I'm sure I never put the whole set on the same character.