You cant use a petty bar as with the Elise and have MSA approval as the roll bar in the VX has the cut outs for the headrest brackets and isnt considered to be up to the job, the Elise doesnt.The Safety Devices cage seems to be the only one that you can buy "off the shelf". Even so I had to order it and it and it was made to order, they dont keep any in stock. I ordered it from MSAR and it took 2 months to arrive. If you intend to race at the start of the forthcoming season you've left it a bit late.
Regarding fitting you have to remove the rear window and fit a plastic one as the new spars on the cage foul the window. You also have to modify the window surround mount. Its got to be said it wasnt a good fit, I had to bend the middle mounts in by way of a twisted rope and bar to get them to line up, the front mounts werent a problem, the rear mounts (spars) didnt line up well but were easier to tease into place. Drilling 10mm holes through the side beams to fit the froint mounting plates was entertaining!!! You cant fit a soft top nor a factory hardtop with it so I'm not sure how you'll get on with the coupe back, I have a Lotus Hardtops single skin roof bonded on.