Saving a vector graphic for the web

Say if i bought a vector image for use in a header? how would the file be saved, as in what format to save it in? do vectors get used that way, for creating headers? do they have layers too, like can you put together a header with a logo using a vector?

 

At some point it has to get rasterized into a bitmap - when you open a vector in photoshop it will ask you what size you want - you can pick any size or resolution and it will convert to a bitmap at that size. Then it's just a graphic like any other, usually on a transparent background, that you can copy into a layer of your header and manipulate.

There's no way to keep it as a vector unless you make the header in Flash.

 

Thanks photodesign, its helped me to understand much better.