Norm, a virtual book tour is where a book gets reviewed on a set number of blogs during a set time period. For example, if you were touring your book, then it could be on tour July 1 to July 31, and you would have to find bloggers willing to review your book. Some tours don't have all bloggers review the book, but rather they might do an interview or a guest post on a blog.
You can't measure the efficacy at this point. Some authors see a spike in their sales, but those numbers aren't known until months after the tour is done. It's just one more marketing tool that authors have, and with print reviews going the way of the dinosaur, it's a new and innovative way to market your book.
You can do it yourself or hire a company, but if you do it yourself it's extremely time consuming, and if you hire a company, I guarantee you get what you pay for (so if you hire the cheapest company, you might as well throw your money away).
I agree with Trish that you get what you pay for, but having coordinated tours for 18 months now, just because a company charges a large sum of money doesn't always mean they're better. If you're considering a virtual book tour company--just like with any service--you need to make sure they can deliver what they say they can. Don't be afraid to ask for references and get an idea of their process before sending money their way.
I agree with what Cheryl said about asking for references. I recently did a side-by-side comparison of a tour I did compared to another tour that another company did, and I was appalled by all the dummy blogs that were used. So I would ask a company to really show you how good of a job they do.