The Vibro Champ circuitry went virtually unchanged throughout its 19 year history. So in theory amps made in different years will still sound pretty much the same. This isn’t always the case, however. In its quest to produce cleaner-sounding amps, Fender in the 1970′s upped the internal voltage of many of its amps, the Vibro Champ being one of these.
The silverface amps in general have a slightly different tone than their blackface counterparts. Silverface amps are cleaner and don’t distort as much. For the most part the differences between blackface and silverface amps are subtle. The age, amount of use, and type of speaker (if replaced), as well as capacitor drift, and type and age of the tubes all have an impact on the tone. Thus you’ll find great-sounding and mediocre-sounding Vibro Champs from any era.