I've breathed fire with MCT oil (the storebought stuff that is basically liquid coconut oil). It's somewhat irritating, and I imagine if deliberately inhaled chronically it would cause harm.

C8 is really fascinating because, I reckon, it's too short to set off the inflammation cascade directly, so it would be up to macrophages to alert the body "hey this doesn't belong". However you still have the usual risk from hydrophobic product in the lung.

Personally, I would use vegetable glycerine and nothing else if you can at all avoid it. PG is a bit more irritating than VG, but any short polyol is going to be diffused away into the mucous much more easily than hydrophobic products.