I triple-box in vanguard using Keyclone and KaVoom-KM (you don't need the KaVoom-KM but it really makes it nice moving between computers.) First of all, the launcher in Vanguard does not prevent multiple instances from running (but you have to have multiple accounts - no running chars on the same account). But you do need to be in WINDOWED mode for this to work. Using keyclone, you will use the ADD button on the main window to Keyclone. I use two computers, only because I like to run my main char alone, and use the second computer for the multiple instances. You have to manually add the VG instance to Keyclone. You cannot use the "auto" function in the general setup. So you would click on the Add button then click in the middle of the game window. You do this for each instance you are running. If you use the maximizer, you right-click on the instance in the Keyclone window and select the region you want it to be assigned to.

This link will show my windows when running. Notice that on VG, Keyclone cannot strip the window borders like it normally does with other games. This is a VG attribute (something about running windows on top of windows is best I can describe it)
http://solidice.com/keyclone/user_ss/ss_vanguard_3_in_1_boxing.jpg