On a multifamily fit-out, the choice between RTA and assembled vanities is almost always about ocean freight per unit, not factory price. The same factory often produces both for roughly the same FOB. The difference shows up in the container.
A 40-foot high-cube container fits roughly 60 assembled vanities with the cushioning and bracing needed to protect finished edges in transit. The same container fits 120 to 150 RTA flat-pack vanities stacked on pallets. Same product, twice the units, half the freight per unit.
That is why developers running 40 units and up almost always go RTA. The freight savings are real, the assembly cost is predictable, and the damage rate in transit is lower because flat-pack ships in tighter, more uniform geometry.
Assembled vanities have a place: short-timeline projects where on-site assembly labor is not available, single-unit fit-outs, and high-end residential where the developer wants the install crew focused on finish work, not box-opening. For everything else, RTA is the right answer on landed cost.