How should I go about fixing a holey laundry room?
The vent flaps are easy to open (by design, it has to open with air pressure) so birds and rodents could easily open them up and make a home. Birds are relatively easy to evict, but rodents are likely to chew their way through the duct and find their way into other places. As for the crawl space vent, I wouldn't want a possible intake vent so close to to the dryer exhaust, it's a humidity issue waiting to happen. So because of that, I'd go for the roof vent. Try to clean it out yearly, and make sure to clean the lint trap out of the dryer between each usage.