Open the dishwasher door and locate the vents on the top. The vents on the outside of the door have small fins, and the ones on the inside are flush with the surface of the door.
Clean off any debris you can see in the vents on the door's exterior, using cotton swabs sprayed with a spray lubricant. Stick the cotton swabs as far into the vents as you can until they come out clean.
Place the end of the hose from a shop vac against the vent on the inside of the door. Turn on the shop vac and move the hose over the entire surface of the vents. Insert a cotton swab from the exterior of the door and move it around to loosen stuck-on dirt.
Vacuum the exterior vents the same way you did the interior ones.