Not really a vacuum seal, the LM was dried out/solidified and not liquid anymore. Since the LM sticks so well to metal, but is itself a 'weak' metal, the film of LM is what is being broken to seperate the parts. I'm not too sure how it changed to that state from how it comes out of the syringe, but the 2 surfaces were 'dry' after taking the block off.

I tried to clean the LM off afterwards with alcohol, but I think sanding is the only way to really clean it off. Thankfully you can layer the LM with the same thermal transfer performance as with a single layer it seems. A fresh coat, quick check of the contact pattern between the block and cpu to make sure I put enough on, and my machine was good to go.