Please help! Feral rescue in Vent!

I rescued 2 stray/feral? Cats 2 days ago. I've been sitting with them, talking to them and trying to get close. One of them was acting a little odd, and the person who told me about them thought one might be injured, so last night I threw on some gloves and took her out. She spooked and took off. I found her behind a large piece of furniture, but she was scared so I left some food, close off the room, and decided to wait. This morning I go to look for her. The food hasn't been touched and I can't find her in the room. What I did find is a open vent higher in the wall. I'm afraid she jumped on the desk and into the vent. How do I get her out? I'm in my parents house... it's cluttered and 3000 square feet and she's a quiet cat so taking the duct work apart is not my favorite choice.
Ive got her baby cat still in the room in a kennel. Will she come back for him?

Yes, She will come back for her baby, if she can find her way back to the room, ESPECIALLY if you can get him to cry a little.  Sometimes they will cry if you just pick them up (I'm not asking you to hurt the little one).  It's more likely that when the house is quiet like late at night the baby will call and the other will answer, or the mother will call with little trilling noises and the baby will answer. 

A friend of mine has a recorded baby kitten crying which she used to lure feral mama cats.  I will write and see if it's a file she can send or post so you can play it.  Also, if mom cat gets lost and stuck behind some wall, she may also start to cry and call so you can locate her.