I am currently referring to the codelab(https://codelabs.developers.google.com/multimodal-rag-gemini#0) which uses

from google.colab import auth
auth.authenticate_user()

to authenticate.

I also see genai reference implement as such

import google.generativeai as genai
import os

genai.configure(api_key=os.environ["GEMINI_API_KEY"])

How can I authenticate when using vertexAI?
I have access to API key and service account created with required roles.