Once a pipe is configured as a vent, it cannot become a drain without reconfiguration. You can connect other vents to this line to minimize roof penetrations, though. You can run a new line out into the yard to connect to the line to the septic tank, though, so you don't necessarily have to go through the floor - you might be able to go out the wall below the frost line and trench to the existing line - it doesn't need to be a home run on its own. It's hard to guide you without being there. You might want to pay a plumber for a consult, even if you're doing it yourself.