How hot will the chimney pipe get (single layer)? I need to decide on what sealant to use between chimney and vermicrete. There's flexible silicones that can go up to 550F. If you need to get hotter then there's sealants up to 2000F, but I believe they will set rock hard.

Otherwise, I need to get creative with a ceramic rope, that seems a fair solution for smoke-proofing (eg. sealing the inside of my vent against the dome), but less so for weatherproofing exterior seals, right? What are my options?