Why do bats sleep in the day time. Do they hang in a vented area and fly off at dusk?

Bats are adapted to hunt at night. This is because bugs tend to fly at night and provide an abundant food source while predators such as hawks, falcons, eagle, etc. don't hunt at night. Bats have poor eyesight (blind as a bat) -- they can find their food at night because they use sonar, that is they makes sounds and listen for the echos and they are very good at this.

Because they hunt at night they have to rest during the day. I don't know about vented, but bats perch and sleep in secluded places to avoid predators. They hang upside down because they can't fly from a standing start, hanging upside down they can release their grip, open their wings and they're flying almost instantly.