Blues fans had been expecting the Belgian stopper to return to St Andrews following Maik Taylors sending-off at Portsmouth on Easter Monday for which the Northern Ireland international has to serve a suspension.
However, Blues boss Steve Bruce allowed Vaesen to remain at Selhurst Park and promoted young keeper Colin Doyle to the first team squad for last weekends game at Arsenal, the first game of Taylor's two-game ban.
Palace are cuurently one of the favourites to claim an end of season play-off place and their boss Ian Dowie has confirmed that Bruce has given the Eagles permission to use Vaesen in those games should the need arise.
Taylor serves the second of his two game ban this weekend when Blues entertain Liverpool at St Andrew's in their final home game of the season but he will be available for selection for the last game of the season away at Blackburn Rovers the weekend after.