Comparing VPS to a Dedicated Server is like comparing socks to shoes (in my opinion).

Much of the comparison is going to depend on the specs of the hardware, what type of processes are being run, and at the end of the day, how much money is being spent.

If you need a shell environment with root access to compile apache or additional software and don't need much power, then a VPS is ideal. Usually with VPS systems you are sharing the system with 8-20 other people. So you only get a portion of the system resources. You usually get a small arount of memory (which can BURST to larger amounts if nobody else is using the memory at the time).

On a dedicated server, you get FULL access to everything. Again, depending on the money spent you can get a dedicated server that is actually weaker than a VPS. Don't buy an Old 1.2Ghz Celeron with 512MB RAM unless that's all you need. The $50-80 is hardly worth it.

For about $200 you can pick up a quadcore xeon with 2GB of ram and large disk space, but again, do you need that much power? If not, then a VPS might be the route to go.

VPS and Dedicated are two very different environments due to how the process and specs are setup. It would be no different than comparing Shared Hosting with VPS in many instances.