I am currently using Vault, BOSE, and MultiInv. Is it a seamless transition to move over to SDFE, or will I need to wait for someone to create a conversion tool?

I know Vault uses the database, and that's what SDFE uses, but BOSE apparently keeps some form of log outside of the database, so how well would that transition?

You can use SDFEconomy's built in convert command:

https://github.com/omwah/SDFEconomy/wiki/Command-Reference#wiki-convert