We know the Super Saiyan transformation power level multipliers, according to Toriyama:
SSJ - 50x base
SSJ2 - 2x SSJ (100x base)
SSJ3 - 4x SSJ2 (400x base)
We know Vegito in his base form was far superior to SSJ3 Goku when he fought Buu, so we can estimate fusion probably gave a power level increase of something like 5x Goku's SSJ3, or a 2000x increase from his base form (or more). Vegito then turned Super Saiyan giving another 50x (100,000x).
Super Saiyan Blue Vegito is probably somewhere along the lines of 1,000,000+ times stronger than base Goku, given how strong SSB has been shown to be. At this point the power levels are simply unfathomable, if they weren't already. I would suggest stop worrying about them and just enjoy the ride. Super has never been consistent with power levels anyway.