[BugFix] Propagate prefix to backend

[azahed98]

Purpose

Fixes #27045 - propagates the prefix attribute to VllmBackend at different callsites.

Test Plan

TBD

Test Result

TBD

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[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @azahed98.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork