From what i know substance files can’t streaming, so the full amount of MB is always loaded into your vram. You can reduce the vram usage and make substance textures streaming by baking them out in the editor, there is a option somewhere in the editor preferences.
For testing purpose i would create a simple color material in UE4, than simply align your color material to all your props “meshes” basically replace all the substnace textures you are uisng in your scene by it.
After that check you vram usage again.