A single vanity is normally chosen because the wall is short, then fitted out as though the wall were long. Under about 900mm there is only one obstruction inside the carcass — the trap and the pair of supply stops — and it sits wherever the last renovation left it. If that is the middle, the cabinet has been divided into two useless quarters, and no amount of internal fitting out will recover what that costs.
Move the waste first, then plan the cabinet. Shifting the basin toward one end and taking the trap with it gathers the pipework into a single bay. What is left over is one uninterrupted run of full-depth storage, which is the only way a 600 or 750mm cabinet holds anything of size. Relocating a waste 200 or 300mm along a wall is a minor item while the wall is open and a considerably larger one afterwards, so it belongs in the first conversation rather than the last.
Once the plumbing bay is fixed, a narrow unit gains capacity by growing upward rather than outward. A single vanity does not have to stop at counter height: a slim tower carried up the return wall above one end, or a bridging cupboard over a lower run, uses wall that a short vanity leaves empty. The toe recess is worth questioning too, since a shallow bank at floor level recovers height that a solid plinth simply encloses.
There is a point below which none of this rescues the cabinet. Take a 450mm carcass, subtract two 18mm sides, the runner clearance and the width the trap needs, and almost nothing remains. At that size a wall-hung or pedestal basin with a proper cupboard elsewhere stores more, not less, because the storage moves somewhere it can be full depth and full height. Both beat a vanity that is really a box around a trap.