Can I put an 85,000 BTU portable propane heater in my basement? It's pointed towards the vents in the house.?

It's risky. Carbon monoxide is created when oxygen levels drop. portable heaters have no vent so carbon dioxide builds up causing carbon monoxide to be produced by flame of heater. I would crack a couple of windows, unless you have a drafty home. If you have carbon monoxide detectors you should be fairly safe.