Bear in mind theres a big difference in the hardware that VPS providers use. A quality VPS can provide better "bang for buck" than a Dedicated Server.
i.e. A lot of budget VPS providers stick 20 VPS on a low end Dual Core box with 4GB Ram, which is OK. On the other hand there are providers who guarantee a much lower ratio, i.e 10-20 VPS per node on a 4 or 8 cpu/core system.
If you have a lot of database-driven pages (e.g. forums) then you need to be careful comparing VPS and dedicated. We've moved some big forums off cheap dedicated servers with only a single 7200 RPM drive onto a VPS with the same amount of RAM and a (shared) RAID 10 10k SAS array and the performance improved massively.
We have sites on VPSs that have taken substatial Digg attacks too, which is cool as they have a low level of backround traffic and then get large peaks of traffic every few weeks when they get on Digg, BBC News and other sites.
It all depends on the spec of the dedicated and the spec of the VPS host server, and the number of VPSs you might be sharing with. As has been already said, at the same price point the VPS could potentially provide better performance and reliability as you don't get enterprise-class dedicated servers with RAID arrays, 8 CPU cores and redundant power and fans on cheap dedicated servers.
Node like Dual Quad Xeon, 8-16 gb ram & you get a top end VPS from it, will probably give you much better performance than dedicated server in same price. Yes! It also depends on the VPS companies how are they managing their node.
Thanks friends. i made my decision. i will buy a dedicated. Because i will install VbSeo to my forum and i will build new websites. i think 1gb ram and P4 2.8 Ghz. i think to buy dedicated from LayeredTech. How do you think about LayeredTech company