does anyone know of a way to select multiple subsidiaries at once when creating a vendor instead of adding 1 by 1? adding say 20 subs manually 1 at a time is just... 😢

Do a csv import vendor-Subsidiary relationship record type

Note that when you create a vendor record 1st subsidiary would be the "Primary Subsidiary". Vendor needs to have a primary vendor in order to create the vendor record. Once vendor records created you use csv to update vendor record. Also note that currently primary sub cannot be changed once assigned, there is an enhancement request pending for that.