Is it possible to overwrite the vendor? We have a few vendors that have changed names and it would be fantastic if we could change the vendor id in order to keep the history!

It's possible to change the vendor name with Adagio Payables, but not the vendor number. You can edit the name and leave the number unchanged, and that won't lose history.

If you wanted to modify the vendor number, then the Adagio Toolkit from Dakota Software has a utility for 'vendor number change'.