So every soulbound item on every character is added to your account-wide transmog list. As someone who's been collecting cool looking items since vanilla, here are my recommendations for dealing with this -
1) Log in every character to add every soulbound item to your transmog list (this may not actually be necessary, but just to be safe)
2) Start vendoring -
- If it's soulbound, sell it unless it has some sentimental value, flavor text, or is no longer in game (because that's just cool)
- If you're missing the appearance, equip it to bind it, then vendor it
- If you have the appearance and it's not soulbound, consider if it's worth selling on the AH
You want to avoid a situation where, for example, you may have two Glorious Breastplates that are not soulbound and you end up equipping both.
Vanilla PvP items remain unavailable for t-mog unless you reached the rank of Knight-Captain, in which case you can buy "replica" items from Area 52 in Netherstorm. You must have reached Knight-Captain in vanilla to buy these items, you must have the Legacy achievement, and only the char on which you reached Knight-Captain can buy them (even though the achiev is account-wide -- bit of a mess, here). These replica items can be used for t-mog for all your chars, assuming they could equip the item (cloth can t-mog the cloth items only). BC and beyond PvP items are available for t-mog normally.
Lots of Draenor crap became toys, so keep an eye out for that as well as you prune your inventory.
Edit - be wary of flavor items from events. They will be soulbound, but NOT available for t-mog, so you probably don't want to vendor / delete them. This is a huge oversite on Blizz's part, imo.