Why?

Because the now that the strong advent corvettes are in the game the developer's vision is now better realized.  Any balance changes to these lil guys should be done after the vasari are in game so that we can see everything.  It is plain to see that the strength of the TEC corvettes lie in their economy, the advent's in their potency/cost. 

I will go out on a limb here and predict that the vasari corvettes will be more durable(at a higher cost, probably around 150-200 credits, slightly longer production time) and have one extra upgrade compared to the other corvettes in the game.  Advent would still be the corvette kings with their early synergy, yet all the races will have more options.  If the TEC get a very strong lucky early game economy, they may just outspam the opposing force to a large degree.  If the vasari get lucky with their neutrals they can really lay down the pain with a corvette rush on the TEC early on... the vasari have always had trouble gaining early pressure on the TEC.  The advent will still be difficult for the vasari to rush with corvettes as long as the discordia is around.  So the game will maintain its fundamental balance and add a little extra to the vasari matchup.

Who knows, the vasari corvettes may even have armor!(definately would need a higher cost though).