You normally store the VA in the ALBUMARTIST or ALBUM ARTIST (notice the space) field. *Note depending on your player it may read one or the other. So to fill the ALBUMARTIST field with your ARTIST field with whats there (in your case 'VA') do the following.
Begin Action Group _Script Test#TEST
Actiontype 5: Format value
Field ______: ALBUMARTIST
Actiontype 7: Import tag fields (guess values)
Source format __: %title%
Guessing pattern: %artist% - %dummy%
End Action Group _Script Test#TEST (2 Actions)
If you have these two actions in a group action in the order there shown then it should work no problem. If you want you can just use the #2 action but you would be safer filling in the ALBUMARTIST field with 'VA' as its helps when sorting your music in various players.