Wolves striker Vokes and Norwich midfielder Crofts have ankle problems while Aston Villa centre-back Collins is struggling with his back.
Inverness midfielder Owain Tudur Jones has been drafted in as cover for the Dragons.
With the friendly falling during the opening week of the Football League season, and days before the Premier League gets under way, manager Gary Speed had elected to name an unusually long back-up list of nine players should he be faced with withdrawals from his original selection.
He said on Friday: "I know a couple of lads have knocks but they will still come along, so that's great. But we have named a large squad and standby list to ensure we are covered in case of any knocks we may have."
Speed's side have no hope of reaching next summer's Euro 2012 finals having lost all four of their Group G fixtures to date.