For a 36-inch vanity, I would ask the vanity seller for the maximum bowl width, depth and drain-zone clearance before choosing a sink. Rectangular sinks vary a lot underneath, and the underside shape can interfere with rails or drawers even when the top-view dimensions look fine. If you want to avoid any modification, the bowl profile matters as much as the outside length and width.

From a manufacturing and fit perspective, I would compare the cabinet opening, countertop cutout limits, faucet spread clearance, rear deck space, and whether the sink leaves enough material around the cutout so the top still feels balanced. If the vanity brand has a sink compatibility list, that is the safest place to start. If not, I would ask them for the largest approved bowl dimensions and a cut sheet before ordering.