Vacuum wouldn't affect a manny tranny. When you lock your ignition the shifter will normally still move, right?

Mine had deteriorated shifter bushings when I got it, and the shift rods jingled around a lot more than they should. I guess it is possible that they could have gotten stuck on something. I would back the car up on ramps and have a look, aiming for low fruit.

You should be able to get another transmission pretty cheap if yours is a goner. May be able to pull the side cover and fix some internal shifting mechanism, short of a rebuild.