Someone else was complaining to me about UUID issues (github.com/GriefPrevention/GriefPrevention/issues/14) and I've double checked some stuff - usercache.json includes even the names of players who haven't logged in for a while.
By the way, I will mention that I'm running behind bungee (offline mode for server, but bungee is in online mode and passing proper UUIDs as per spigot.yml use-bungeecord option).
By the way, as per your comment about /scs owner storing name if UUID isn't found; it is indeed printing the correct UUID on screen. I guess it is then the latter...
Actually, I just tested this myself, https://github.com/MLG-Fortress/HotFix/commit/23a7644ebe0e7971675263c0f2490369879285ab and indeed it does not return an offlinePlayer with a name, until Bukkit#getOfflinePlayer(string) is used with the corresponding name. So yea - until vault stops trying to get the player's name(shouldn't it be using UUIDs only anyways? Maybe it doesn't because the server is in "offline mode"), you'll have to store the name