When this house was built it had heat & air comeing out of the cement floor, but the ducts started to collapse so we had it installed in the attic. The old unused vents are still in the cement slab floor. We are now going to installe new laminate flooring & need to know how to fill in the old vents to run the new floor over them...

Stuff newspaper into the holes and leave about 2-3" depth between the paper and the floor. Mix up some mortar or sand mix and pour it into the hole. Level it and smooth with a trowel. When it sets you can put down the flooring on top of the underlay.