Six years during the healthiest time for roleplaying is not "roughly contemperous.
Unless I'm doing my math wrong, 1989-1991 is two years.
I will agree that Prince Valiant did not make the same splash. However, the elements that many attribute to Vampire were already out there. White Wolf just found a way to gel them into a system and market them with some other popular elements.
Elements does not a new genre make. Case in point: LOTR. You could call it swords-and-sorcery or you could call it epic fantasy. Even though LOTR used elements that had been used before, it was a genre-making work of art because the gestalt of the elements used was quite distinct. OTOH, Elric was written at a transitional time in fantasy literature, and straddles the line between swords-and-sorcery and epic fantasy in a way that makes it hard to categorize.
Prince Valiant was, in my view, a thoroughly old school game. Although it introduced many innovations and focused on narrative tools for gaming, there was little to set it apart in philosophy from something like Marvel Super Heroes or Gamma World.
The (optional) rules for allowing the players to take over as the gamemaster and run a scene or two was as ground breaking as I remember in those days.
I rmember multi-DM dungeons from as long ago as I remember D&D, which was circa 1983. In any case, one trick or technique still does represent a new kind of game.
Characters in D&D, in Marvel Super Heroes, in Champions, and in, yes, Prince Valiant,
had adventures. But Vampire was about
being a certain kind of entity. Ars Magica comfortably fills the role of bridging between the two styles, anticipating the way Vampire would fulfill certain game play needs but not setting aside the basic fantasy wargaming agenda (events happen in a fantasy world, GM adjudicates the consequences of PC actions). D&D is a warm apple pie, Vampire is quiche, Ars Magica is a mild sweet potato pie. Prince Valiant is just plain old apple pie, with a little spice.