Never had a VDB with Copa, and never even seen them asking for volunteers at any stage. Only "VDB" for an economy passenger I have heard of was an Op-Up to business class

I was on a flight 2 weeks ago to MCO, which was Y0 a couple of days before the flight, with about J11 (could tell from upgrade page). Flight went out full in J, and economy looked busy enough

I would guess that its harder to VDB from PTY for the sole reason that they can always change the plane. If you were to be on the newer 800, and they are over by 4-5, then they could easily switch to the older 800, which has 6 extra seats. Many times I have checked in for one plane type, and then when I boarded it was either the other 800, and once the 700 (that was to either MIA or MCO, and it was maybe 50% capacity)

Please report back though!