FallbackDir: orports: Introduce and use VecBuilder

This is part of #447 (closed) and therefore #371 (closed) and #285 (closed)

Found with

git-ls-files | xargs perl -ne 'next unless m/^\#.*derive.*Builder/..m/^\}/; print "$ARGV:$.:$_"' 2>&1 |less +/':.*Vec'