"Art is that which is made with the intention of stimulating the human senses as well as the human mind and/or spirit." This definition from wikipedia aptly describes what many collectors of vintage art posters are looking for - not just something to hang on their wall, but something that will stimulate the senses, mind, and spirit.
But what exactly is "vintage" art when it is applied to posters? By its nature, most art has a timeless quality, making the term "vintage art" seem almost redundant. When most people think of vintage, or retro, art, they are probably thinking of the types of posters that are from the 50's and 60's. The early advertisements are a great example of vintage poster art. Vintage art may not necassarily be older art, such as a picaso, but rather a poster from a different era. Vintage doesn't have that same undefinable quality that classic art has - and that's what makes it vintage.