Milling the VM's Front ASA
First I started by shamelessly ripping the above pic from Air Gun Dynamics. You'll notice the VM has a HUGE chunk of solid aluminum in the front. This chunk adds unnecessary weight to the front of the gun. However, this also gives you a lot of freedom in the way of customization. Newer VM's don't have this chunk of aluminum because it has been drilled out at the factory.
Just a simple vertical ASA. This would be the easiest to do and would also allow you to go back to the original front ASA. You could also easily go dual tank with this setup.
This setup also adds the internal expansion chamber mod.
If you're looking for a more drastic mod, just chop the whole ASA off! There are lots of different ways to do this, this is just one example. I would suggest a professional custom shop/machinist perform this mod since you could really screw up your VM if you don't know what you're doing.