I only pull vacuum on my solvent if it is far below its boiling point, pulling a vacuum itself. At that point, the only vapors present are the nitrogen vapors. Once those are removed, you can recover more easily.
As far as nitrogen assist from the material column. Once you have pushed your solvent from the material, close off that part of the system and be done. I wouldn’t recommend nitrogen to go through your running pump. It’s not built to compress nitrogen and it can/will blow out the seals on your pump.
Now, if you were removing the nitrogen from your solvent tank, then pushing the nitrogen from the material into the collection, letting the vapors pass to the solvent tank (pump off). Then once the nitrogen has passed from the collection to the solvent tank (material column closed off now), you can close recovery lines from collection, remove nitrogen from solvent tank, then resume recovery, open collection, and turn on the corken.