fix: Change VERTEXAI_LOCATION from 'us-central1' to 'global'

[Hurleveur]

Took me days to figure out :

us-central1 doesn't have access to the latest gemini models like gemini-3-flash-preview, the best you can get is gemini-2.5

As can be seen here https://docs.cloud.google.com/vertex-ai/generative-ai/docs/learn/locations#united-states

Changing it finally allows me to use gemini-3-flash-preview, I tested it and it finally launches. I had wrongly assumed it wasn't possible to use gemini-3!

Edit: screenshot to show it works now:

[greptile-apps]

Greptile Summary

This PR fixes the example VERTEXAI_LOCATION value in the Vertex AI provider documentation from "us-central1" to "global", enabling access to the Gemini 3 model family (gemini-3-pro-preview, gemini-3-flash-preview) which are already listed as the available models in the same document but are not available in the us-central1 region.

• The fix is correct and consistent with what Google's Vertex AI documentation states about regional model availability.
• The global endpoint is now the documented default — users in environments with strict data residency or compliance requirements may wish to choose a supported regional endpoint instead, but that nuance is likely out of scope for a getting-started guide.

Confidence Score: 5/5

• This is a safe, correct documentation-only change with no code impact.
• The change is a single-line documentation update that aligns the example configuration with the models the doc already advertises. It is backed by Google's own regional availability documentation and has been manually verified by the author.
• No files require special attention.

Important Files Changed

Filename	Overview
docs/llm-providers/vertex.mdx	Single-line documentation fix changing the example VERTEXAI_LOCATION from "us-central1" to "global", which is required for the Gemini 3 models already listed in the doc's Available Models table.

_{Last reviewed commit: 3f469ad}

[0xallam]

LGTM!
Thanks! @Hurleveur