In Settlers of Catan, if you have unrevealed victory point cards that would make you win, are you forced to declare victory?
You have won the game as soon as your have 10 points on your turn, there is no requirement to "declare" victory. So no, you cannot keep playing extra rounds. From the official FAQ:


  Victory Conditions - May I continue scoring points after having reached the number of victory points required for a win?
  
  
    This could make sense in a private tournament where the scores from several games are added up. For individual games and official tournaments, the following applies: You have won as soon as you have reached the required number of victory points on your turn – and then the game is over.
  


http://www.catan.com/faq/4613-victory-conditions-may-i-continue-scoring-points-after-having-reached-number-victory-points

and:


  Victory Conditions - What if someone has reached the required number of victory points for a win but is unaware of it? Did this player win anyway?
  
  
    Yes. The game ends as soon as a player has reached the required number of victory points on his turn. If he’s unaware of it, you should tell him, because you can’t take an already gained victory away from him.
  


http://www.catan.com/faq/4616-victory-conditions-what-if-someone-has-reached-required-number-victory-points-for-win

Note that you have the point from VP development cards as soon as you buy the card. You don't "play" the development card to get the points; you already have those points.