It might help if you could post pictures of the vanity.
One thing to consider is support for the weight the stool has to hold.
You might want to consider a rolling stool that fits into the cabinet when not in use. This depends on how the cabinet was built and if it can be modified to accept the rolling stool.
Another thing would be if the plumbing will be in the way and needs to be re-plumbed to to allow the stool to fit.
Please post pictures of the vanity and you will probably get other responses.
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