First off if using ASP, that's serverside, and it's usally good to work with VBscript on the serverside as it offers most flexibility in ASP, however on the client side things are different.
both have their limitation, but I would concentrate on using javascript if you are going to write client side scripting, as VBScript's major flaw is that it only works with InterNet explorer, no other browsers, javascript works with almost all browsers.
I write in ASP, and I found it best, to write VBScript serverside, and Javascript clientside.