Well here are my findings. Your sample script you include has an error, You stated this:
Run('"c:\vpinmame\vpinball.exe" -Play -"' & $CmdLine[1] & '"', '', @SW_MAXIMIZE)
and it needs to be this:
Run('"c:\vpinmame\vpinball.exe" /Play -"' & $CmdLine[1] & '"', '', @SW_MAXIMIZE)
Your hyphen, "-", in front of play needs to be a slash, "/".
Next, your sample splash is named wrong. Not a big deal but some people may not catch it.
Next, you sample Mamewah INI file needs a "1" for the send_rom_path.
All of this is pretty minor. However, I have the same focus issues that I have with my wrapper. If I launch a simple table like 3 Stooges everything works great. However, if I launch Addams Family which has a dialog box that pops up you don't allow for sending of the "enter" key. You will find as you look into this, there is no standard and everyone designs their tables different. I have seen some that require 3 enter keys to actuall get the table to load.
For you own feedback try the table Addams Family from here:
http://www.vpforums.com/vptables/tables.php?ltr=ALet me know how you make out. Maybe I did something wrong.
Good Luck,
John