Is it possible to add a ground to a vacuum?
Not being grounded is not going to affect the way the vacuum cleaner functions. In fact, most vacuums I've had/used are not grounded. They are mostly made from nonmetallic parts and insulated in such a way that grounding isn't needed or impossible to achieve. 

Now, if you're using an ungrounded vacuum to clean electronics, computer motherboards, etc.and are worried about static charges, then by all means you could remove the two conductor cord and install a 3 conductor cord and a three prong plug.