post https://{baseUrl}/V1/storeview//vendor/sign-up-form/validate-email
Use this to check if an email already exists as a user account for vendor.
Sample Curl
curl --location 'https://mygatewayurl.com/V1/storeview/{store-code}/vendor/sign-up-form/validate-email' --header 'Content-Type: application/json' --data-raw '{ "vendor_prospect_email": "[email protected]"}'