I don't know the specifics, but I heard something about if you get a bunch of items without giving anything in return, you'll get marked for real world trading? Don't know if this is true, for example what if someone was giving away items, or you were giving items to a friend, etc.
Also, a VPN will only keep your other accounts from getting banned, if you use a VPN on all your accounts they will have the same IP, meaning they will all get banned. Only use it on the RWT accounts