mulching blades are set up to cut the blades of grass "twice" with the result that the resultant "mulch" can remain on the lawn without concern. The penalty is the need to run at a lower ground speed so the bi-level blade edge can do its thing. If you are vacuuming up the cut grass, the benefit of the mulching blade seems of little value and you pay the speed penalty. That said, if the blades you have are mulching blades, it doesn't seem that you couldn't use them. If there is a any concern with a mulching setup and a vac it is not the blades, but the special airflow inserts on the underside of the mower deck, which might fight the vac air flow.