Hello - is there a way to enable several currencies as a default for all vendors?

Create a workflow on vendor ( event in create ) Use action Set field value for each currency that you want by default on vendor ( trigger on after record submit)

Consider using more vendors tbh, not sure why you want several currencies to be the default, sort of counter-intuitive unless it is for different subs. Note if VAT is in play here you need to be extra careful.

thank you