engine sucking up transmission oil

[marvxyz1]

I have a 1980 gmc with a six automatic. I've been losing transmission oil. I checked under the hood and on the left side of the engine as I am facing it. There is a plug on the side of the engine.with two vac. lines going to it. With the motor running and the vac lines connected, I get transmission oil leaking from it. I also seem to be getting transmission oil sucked into the engine, as I get gray smoke coming from the exhaust, when I start the engine. what is this plug called and what would be the fix for this problem?? I would appreciate any info you may have on this problem, Thanks for your interest..