The problem with a vertical shooter in MPAGD is that, as standard, MPAGD's LASER command only accepts LEFT or RIGHT as an argument, meaning Lasers can only be fired left or right. To create a vertical shooter you would need to use SPRITES for your bullets. This means you would need to keep a close eye as to how many sprites are on screen (max 12) at any one time. It also means you would need to write some code to handle hit boxes, or perhaps hack the collision detection routine inside the MPAGD engine.
I have a simple prototype vertical shooter I am working on and, if successful, will write it up as a series of tutorials on my blog.