Welcome to the Bunnings Workshop community. Thank you so much for joining us and sharing your question about replacing a vanity top on a non-standard-sized cabinet.
A few conditions must be present for your plan to work. The vanity cabinet must be structurally sound and has not been water damaged. There must be enough overhang space to the left and right of the vanity for the Mondella Rococo 900mm Ceramic Vanity Basin With 1 Tap Hole to fit in. Ideally, you want the new top to be centered to the cabinet, make sure that the bowl does not hit the drawer assembly of your vanity cabinet. If the drawer assembly is in the way it will result in the top being offset and will make it look unbalanced.