From this paper, my read is that the vacuum doesn’t substantially affect the decarb time. There are time/temp graphs about half way through. They look not dissimilar to ones from non-vacuum processes.
I’m not a chemist, but my understanding of decarboxylation is that we’re basically dumping enough energy into the material to beak the bond between the carboxyl and the cannabinoid. I think that means that the primary determinant will be the heat energy applied to the material, not the pressure of the environment.
It’s definitely possible I’m completely wrong on this one and will be happy to be corrected by someone smarter than me on this one.