I'm currently using MultiInv but I want to go back to vanilla Minecraft (not at all because of MultiInv)
Unfortunately I'm a bit locked in right now, because I need the information in MutliInv's yml files to do so.

Could you possibly add an export command which could export the player data to Minecraft's .dat format?

Thanks in any case for your work!

Close down all other worlds and then players will be "forced" to join the main world, which, at that point saves the files to the main .dat file. After a week, switch over to vanilla.