The trouble with VPS hosting is that it is liable to wind up just like the shared hosting industry, vastly oversold and overfilled.
People tend to frown upon low quality shared hosting because you never know if there is enough resources available for your account to get through. If you split up CPU, Memory, and Disk I/O into shares and say that there are 100 shares of each available at any given time, well if account1 is using 60 shares of each, and account2 is using 40 shares, then there's nothing left for account3 to draw from.
Unfortunately, you're doing the same thing with a VPS. There is a node that houses all of the virtual environments (VPSs). That node has a fixed amount of resources. Again say that the node has 100 shares of CPU, Memory and Disk I/O, then if vps1 is using 70 shares and vps2 is using 30 shares, then vps3 doesn't have any shares to use.
Now obviously not every provider is going to be like this. Cheap and low quality providers will aim to stuff a server as full as possible to make as much money per server (or node) as possible.
So if you find yourself with poor performance on a shared hosting provider, don't necessarily think that moving to a VPS is going to help. If you are on a cheap, low quality (I admit, this is subjective - I don't know how you quantify this) shared hosting provider then moving to a cheap, low quality VPS provider may not net you any performance gains. Moving from a low quality shared hosting provider to a higher quality shared hosting provider may provide better performance improvements.
Likewise, if you are already on a high quality shared hosting provider and need better performance, then moving to a low quality VPS could result in even worse performance.
Everything is always going to be relative. Moving to a dedicated server is the only way you are going to know that you have dedicated resources up to the limit that that hardware provides. But then you have to look into how the datacenter housing that dedicated server is put together. How much bandwidth do they have available to you? This is probably less of an issue unless you are Google or Amazon.