hi spacer
as far as i know, vst is the format of softsynths you use in cubase, logic, fl, reason etc and rtas in pro tools.
with an vst to rtas convertor (or vst wrapper) you can run vst's in pro tools.
rtas and vst softsynths work exactly the same way. you can see it as a word-document: you can save it as .doc or as .rtf. It are different extensions but it's the same document. I've heard that rtas is "better" than vst, but i'm not sure about this.