This thread is a bit dated but I thought I would pop in and say I have the exact same problem.
When I first put the igo-l on my vamo it worked fine, but after taking it off once or twice, it no longer fits on the vamo.
It will fit just fine on any of my mech mods and my eGos but it takes a lot of fiddling to get it on the vamo and even then it usually just falls off.