Keep firmly in your mind the sort of assets that you require to serve your site(s) when seeking a host. Cost in a VPS is important, but not as key as you might think. VPS resource availability is scalable, so the cost that needs to be looked at is comparable cost from one host to the next. Also – as cPanel revised their pricing model recently, web hosting companies across the board will have to pass those costs on to users sooner or later. You will need to consider the cost of control panel when selecting a VPS plan. Companies like ScalaHosting has developed their own control panel to mitigate this issue – so their users would have little issues with the price hike.
Essentially, here’s the criteria I would use to judge things – if your site is made up of primarily static, HTML-based content, then you probably don’t need a VPS package.  However, if you have a large amount of files stored, multiple sites, dynamic content, and the possibility of major traffic from time to time, then you might consider upgrading to a VPS. It’s a powerful package that allows you to do more than you could with a shared hosting plan, but requires less investment than a dedicated server.
Max Ostryzhko:
A VPS doesn’t just have more RAM, disk space, and a  proprietary share of CPU than a shared account. Depending on the provider, some VPS plans offer burstable memory, which is a pool of RAM set aside for extraordinary events. This is the stuff that can help counter the so-called “Digg-effect,” that much-whispered about occurrence that’s the simultaneous hope and fear of everyone who runs a web site. When you have an unexpected high traffic event, burstable memory will call on a pool of reserved, shared memory to satisfy the needs of temporary high traffic. This is not available on shared servers and, while the necessary memory is available on a dedicated server, your site might not get the kind of traffic on a daily basis to justify the expense of a dedicated server. Again, not all VPS plans have burstable memory, so ask your provider if their VPS plans do.