hey,
ah well any speaker will perform better than the standard speakers, as i have a vy also...although you will not get the full potential of any speaker off of the stock headunit because of the crappy amp of the thing!!
i suggest go for a set of 6" components (with tweeters) as this will give better sound quality....a good set of 6" coaxials in the back doors....or other option is 6 x 9 's on the parcel tray...depending on your DOOF DOOF liking :P...
if not satisfied the next step would be...changing the headunit to something with rca outputs for an amp to hook up the the front components...as this will release the full potential...
i have recently changed the VY headunit to a double din pioneer...and even the 4 x 50w amp compared to the crappy 15-20w blaupunkt gives a real difference at higher volumes...and i have also gone the 6 x 9 option...
my next step is to now install a pioneer 2 channel amp i have lying around 2 some new 6" components i am purchasing and then a Mono block amp for 2 subs
hope this helpssssss
good luck