At low level you can stuff a key into the keyboard buffer.
A low level DOS/Windows type program can be call from
VB.
If you stuff Alt-tab you could swap to another program, just like hitting the keyboard.
Is it really worth the effort? Just use Alt-tab to swap to the program.
Windows will do all the hard work.
Start the camera program before Mach3.
Caveat:
Mach3 might not work properly if you steal focus from it (and especially the keyboard), but if you are not running G-Code at the the time who cares.
If you don't run Mach3 in full screen, you should be able to see at least part of your camera view, if the software driving it updates in the background.