I was thinking more about an op-up on an international flight vs. a VDB.

If DL overbooks and there are only 4 Y seats left, then I think by definition the flight is oversold if DL is willing to oversell by 4.

I realize this is dynamic situation but sometimes I have a choice of taking one international flight vs another and the chances are much better for an op-up with a Y4 flight vs. a flight with U fares still being sold.