All customs/immigration is done at YVR as a pre-clearance airport. You will arrive at ATL and enter the main terminal with no additional processing just as though you were on a domestic flight.
This is true for most flights out of YVR but not all. Pre-clearance closes at 8 PM at YVR. The YVR red-eye to ATL operates as a regular international inbound flight. OP will clear customs in Atlanta.