A very unique idea, I'll give you that. I think it would definitely be difficult to retain the actual vaporizers. How will you make sure all of the irresponsible customers, especially in your case when smoking usually equals irresponsibile laziness, return your products? You'd have to rely on a credit and debit system so you can charge them in the event of a lost or stolen item. Which interfers with my next point:

In my opinion, if someone wants to rent a vaporizer for a month, in which you'd probably charge well over a hundred dollars, that person would be more likely to buy a vaporizer themselves. You'll be getting a lot of walk in customers that only want a day or so just to get blaaaazzzed with all their friends sometime, and those people will definitely want to deal in cash and little paperwork.

Not to mention all the paperwork you'd have to maintain. I'm not saying it won't catch on and make you bank.. hell, if you rent out vapos at 10 bucks a day, a sure-fire sale, you would have your money back in a few weeks. But it's not going to be as simple as you imagine.

EDIT: Okay, I just realized you meant starting an online business. I'm a little too high to compare in my mind whether or not my points are still valid, so I'll leave them up.