I am building a deck attached to the house, and on one side I will need to have the ledger covering a section of wall that has a crawlspace vent cover. If I have a big enough gap between the ledger and the siding (maybe bigger than what you would normally do -- make it closer to 3/4" perhaps?) would that be enough space to let air escape through the vent? Or is there some other way to make sure I don't block off the ventilation?
I would leave a gap in the ledger where the vent is, then cover that gap with a shorter piece of ledger board that overlaps, and is lag screwed to, the main ledger. This will give your crawlspace vent 1 1/2" of breathing room, which should be fine for letting your crawlspace breathe.