Does the basin have wall fixing holes for the fischer type basin fixings?
I would use them as additional support, as well as screwing the vanity securely.
Put a good bead of silicon along the contact areas of vanity and basin and basin and wall.
You could consider using a pair of basin fixing brackets to support the basin if no screw fixing holes, as well as silicon seal to vanity and wall.
Really depends on basin type and vanity unit, there are many variations of each.
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