It's kinda weird that forced fullscreen or true fullscreen actually works in VPX together with DB2S and VPM. It seems that todays graphics card drivers can handle such case in a different way like it was when DX9 was the latest DirectX version. On old systems you would get a lost device error message if you started an application in true fullscreen and do a task switch with Alt+Tab. When switching back to the full screen application the application itself has to handle that case otherwise you got a black screen.
Now with Win7 or higher and latest drivers this problem seems to be solved but I would say that it's more a coincidence
If you do a task switch by pressing alt+tab you see that VPX is still the selected process. It looks like DB2S or the .NET framework that drives db2s "steals" the access to the frame buffer so it can render it's stuff to it. When switching the tasks back and forth you will notice that the system needs some time until you see the playfield again because the driver has to reinit the frame buffer for VPX again or handle the sharing between the different applications VPX, DB2S and VPM.
So a lot of guesses here but adding a simple focus switch like you would normaly do for windowed applications doesn't work here. The application still has the focus but not the access to the frame buffer while DB2S starts. I searched the net up and down for such case but couldn't find anything that is usefull for such case.
So you only have two options:
1. don't use true fullscreen
2. use it together with the AHK script and potential problems that might come up with different driver version and/or systems. There is nothing we can fix in VPX at the moment.
That might change if we add some kind of DB2S into VPX itself but definitly not for VP10.1 