Legacy patcher re-install the High Sierra drivers (I think?) for the now unsupported graphic cards.That makes sense, if you assume that dosdude's legacy video patcher (or some other patch) *restored* the analog signal... DVI=>VGA *did* work using the pre-Metal card. Now that I have the metal card (with your kind help) I've disabled the legacy card patcher and experience the blanking.
Apple don't have a Mac with VGA output since El Capitan, when MP3,1 was last supported. VGA support using the analog signal of DVI connections was finally removed from the Mojave GPU drivers. Active VGA adapters still work, these don't depend on the GPU drivers.
Legacy patcher is incompatible with METAL GPUs.