Hello,
Your first step in filing a grievance with your guild was the proper step.
I have seen this issue before, and it honestly comes down to the CBA. A request that has been approved for which you have relied on is a new element to your employment contract.
However I have seen CBA's that allow for rescission of approved vacation requests, with double time and a half, or some other form of restitution but always with at least 2 weeks notice, so plans can change etc. Never with 12 hours notice.
If there is no restitution, and there is a play on the word "request", you are correct insofar as request is a unilateral "question". An approval is an "agreement". Agreements can only be rescinded under the terms provided. If your CBA has terms governing how these "agreements" can be terminated, those terms by the nature of you being employed there would form part of the agreement.
If you have the approval in writing, I would say you are in an excellent position to keep your vacation.
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Regards
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