I don't know your situation or where you live (both important factors) but for myself living in my L300 full time a combination of keeping the front windows cracked at night and not keeping closed up constantly is plenty to keep the inside of the van comfortable and not a swamp. Having rain guards on the front windows is almost necessary so you can keep those windows open no matter what weather, and if possible protecting your side windows from rain as well.
Currently my van has rain guards and sliding rear windows and I don't feel like I need a roof vent.
Just my 2 cents.
Allen