Update README.md to show API key usage for Vertex (#3060)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

1 files changed, 14 insertions, 0 deletions

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@@ -51,6 +51,20 @@ The Gemini API provides a free tier with [100 requests per day](https://ai.googl
 
 3. (Optionally) Upgrade your Gemini API project to a paid plan on the API key page (will automatically unlock [Tier 1 rate limits](https://ai.google.dev/gemini-api/docs/rate-limits#tier-1))
 
+### Use a Vertex AI API key:
+
+The Vertex AI provides [free tier](https://cloud.google.com/vertex-ai/generative-ai/docs/start/express-mode/overview) using express mode for Gemini 2.5 Pro, control over which model you use, and access to higher rate limits with a billing account:
+
+1. Generate a key from [Google Cloud](https://cloud.google.com/vertex-ai/generative-ai/docs/start/api-keys).
+2. Set it as an environment variable in your terminal. Replace `YOUR_API_KEY` with your generated key and set GOOGLE_GENAI_USE_VERTEXAI to true
+
+   ```bash
+   export GOOGLE_API_KEY="YOUR_API_KEY"
+   export GOOGLE_GENAI_USE_VERTEXAI=true
+   ```
+
+3. (Optionally) Add a billing account on your project to get access to [higher usage limits](https://cloud.google.com/vertex-ai/generative-ai/docs/quotas)
+
 For other authentication methods, including Google Workspace accounts, see the [authentication](./docs/cli/authentication.md) guide.
 
 ## Examples