Pics will definitely help, Alec, but I see what you are saying. Problem with what you are proposing is that tile, stone, and CBU is not waterproof, so not only is the joint where they meet the vinyl and the concrete suspect, but the space in between is too. I am going to show my ignorance of general house construction (that my friends here know much more about), but I am guessing from the studs out you have 5/8" OSB, house wrap of some kind, and vinyl, right?
I would think that you could board the bottom with a similar thickness OSB, thinset and mechanically fasten a 1/4" CBU, waterproof it with a surface applied membrane, and thinset stone or tile. I am not sure how to treat the top of the stone, though, as it will sit proud of the vinyl, but I believe I have seen a cap block type trim used in this manner with stone. The key would be to ensure that the waterproof membrane applied to the CBU is essentially "behind" the last row of vinyl so that water will shed away from that top joint.
How far out is the face of the slab from the studs??
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Brad

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