No, it’s the opposite effect.  Crown will open the ceiling.  Large base boards with crown and a color of paint on the wall, will open the walls.  You will see a distinction between two points.

Once mouldings are installed, they need to be caulked as well as filling the nail holes.  If not painted, the non sealed caulk will shrink and crack over time.   Temperature changes can effect the moulding as they are made of MDF.  Proper caulking can help prevent this from happening.   

There are 3 different solutions.  Either of the 3 options below, typically, will not reduce air flow.  This is more a personal preference.

1. 1.   We can return the crown next to the vent.
   

2. 2.   We can notch the crown and go over the vent.
   

3. 3.   We can accent the crown with detail crossing over vent. (not recommended but is an option)