I have a vertical pvc waste stack which I want to insert into a cast iron pipe which is flush with the floor. The plastic pipe fits snugly into the cast iron pipe. What can I use to seal the plastic pipe into the cast iron one?

It really needs a no hub band to make the connection. You may have to increase the size of the hole around the pipe to allow you to tighten the clamps on the band.

Unfortunately I can't do that. The cast iron portion is in a concrete floor. I took a plastic pipe out of the cast iron pipe and there was some sort of gray putty-like stuff there. I need something like that.

how far can you insert the plastic? can you even fit a hacksaw blade in between?

Six inches or so. No I cannot insert a hacksaw between the plastic and cast iron. They fit together snugly.

If it's hub cast iron you can get a service weight gasket and a Charlotte 119 fitting

the problem is that code calls for a 'full size' cleanout before reducing.
if it doesn't matter to you, just epoxy things together. install a cleanout tee at 18 inches off the floor and enjoy the superbowl...