VST stands for Virtual Studio Technology. They are just a different, more ubuiquitous form of plugin. There are essentially two types of plugs for the PC world, VST and Direct X. Vegas supports both now. Sonic Foundry was the very first company to seize on the concept of DirectX back when it was called Active Movie, then Active X, now Direct X. This was in version 4 or 4.5, I can't remember which, of Sound Forge.
Anyway, Steinberg developed their own tools, and released the spec. There is no difference in quality of sound, sometimes VST plugs run more lightly on the CPU and other times they don't. Most manufacturers offer both VST and DX versions of their plugs. In Vegas, DX still has some advantages, but they're minimal.