The VMX is well know as a upgrade body for the VM-68. The VMX is also available as a whole unit. The VMX on my Custom VMs page is the only one I've seen [at the time in 1998], although I have heard of others. I bought that one and it was just incredible.
The VMX body is made of aluminum, not plastic as some people seem to think. It has a built in vertical CA adapter and a side tap like the VM Magnum. The gun is field striped with just one field strip screw on the back. The back has a "flip lid" that allows the internals to slide out. The valve still requires the valve tool for removal though. The power feed and anti double feed do not have field strip screws, but you shouldn't have to take them off to often anyway.
The anti double feed seems to work. I've never really had to many problems with double feeding (except when I was using Superball paint with my 16" All American). The power feed probably works ok too, though I didn't notice a difference in performance over the stock feed adaptor on my other VM.
The VMX unit I bought came with some nice extras. It had a polished hammer, TASO venturi bolt, Pro-Line expansion chamber, 10" stainless steel ported barrel, and a Double Trouble Trigger.
The polished hammer was nice. It had a nice finish and performed well. It had no safety on it though which really bothered me. The venturi bolt was ok also. It must have been an older TASO bolt because it had the stainless back on it like the stock bolt. I had a newer TASO venturi bolt and it was much lighter than the other one but had similar performance.
The Pro-Line EC is the EXACT same as the Skan-Line Turbo Boost EC. Because it is a small EC it did not help performance very much. But when you're shooting a VM, any increase in gas efficiency is welcome!
The barrel is a no name stainless steel barrel. It has a smooth bore and looks nice. I didn't shot it enough to get a good feel for it though.
Now for the coolest thing about this gun, the Double Trouble trigger. This is the soul reason I bought this gun. These triggers are hard to come by so when I saw this one I just bought the whole gun. The Double Trouble trigger fires when you pull the trigger and again on the foward stroke. I could easily shoot 8 balls a second and with practice I got to about 10. The only problem is feeding. Shooting that many balls a second requires a fast loader. My VL2000 is ok, but a VL Revolution, which can feed 10+ balls a second, would be better. The trigger pull is also very long which prevented me from getting even more shots per second.
The Double Trouble trigger is much more complicated than even a stock VM trigger. I never took it apart (mainly because I was afraid to!), but I saw that there were additional springs in it. To fire twice, the mechanism also requires two secondary sears. Very weird.
NOTE: to install a trigger assembly in a VMX body requires you to cut off a little bit of the front and back of the trigger housing. Something to think about if you would like to get a VMX body.
After I got the gun I added a .45 grip. With the stock M-16 Lonestar grip, holding on to it with one hand was a little uncomfortable. Also, the trigger pull was very long. Adding the .45 grip made holding and shooting it feel a lot better.
If you would like to buy a VMX body, I&I Sports might still sell them. They used to sell them for $75, but I haven't checked for a long, long time. America's Paintball Superstore might also sell them, but you would have to call to find out.