Amazingly enough, this does indeed seem to be the problem! I quickly tried replacing libvpl with onevpl, which was what I had installed a week ago… and which incidentally doesn’t have any ®:
$ pkg-config --list-all | grep vpl
vpl oneVPL - oneAPI Video Processing Library
And this was sufficient to get cabal build working again! Then, reinstalling libvpl immediately broke cabal build. So this looks like a Cabal bug, which I will report later today.