figured out, I think, that I can programmatically execute Recaptcha and then store the token in the state, sending it with the form. This means the Recaptcha check is only back-end, but that's fine.
maybe I need to re-execute on error though?
this was a bit helpful: https://itnext.io/how-to-use-google-recaptcha-with-vuejs-7756244400da