Description of problem:

ES output with authentication set to 'token', adds strategy="bearer"  to the vector configuration for the ES auth.     Same result whether you use token from secret or SA.

The collector pods wont start, and the resulting error is:  
Configuration error. error=unknown variant `bearer`, expected `basic` or `aws`

outputs:
  - name: es-external
    type: elasticsearch
    elasticsearch:
      index: '{.log_type||"unknown"}'
      url: https://es-service-endpoint:9200
      version: 8
      authentication:
        token:
          from: serviceAccount 

If "bearer" is not a valid option for the vector configuration file,  Jeff commented we should be able to resolve this by using request.header to add the bearer token.