I recently studied VLSM and am relieved that it doesn't seem nearly as complex as I thought it would be.
That being said, I'd definitely like to practice it some more. I was just curious if you've come across any VLSM practice sites? Ideally, I'm looking for a site similar to http://www.subnettingquestions.com which would let me take my time until I get more proficient and can perform speed tests.
I don't know if these are too basic for you, apologies if they are.
To improve your speed in thinking and make your mind translate to/from binary faster, check out the Cisco Binary Game.
I wish I could be some help here but I am not aware of any such sites. To help me learn, I just made up some scenarios, and practiced that way. You could always do the same, and post them up here on CLN, if you want to make sure your answers are correct.