If you have problems with 'wildlife' get some metal netting - the sort that the americans use as 'screen doors' and make covers for the vents - it will prevent butterflies, wasps, cars, dogs, foxes and small children! from getting in. That way if you have an auto vent (wonderful thing!) you don't have to worry - also later in the season you can leave the greenhouse door open, but not have all the problems!

As an aside if you are putting in a high level vent, you should consider a low level vent to get air moving across the plants and reduce the risk of still air moulds such as botritis.