A vehicle frame assembly includes a main frame part including a side sill extending in a longitudinal direction of the vehicle frame assembly. The side sill has a C-shaped cross-section with a top and bottom walls, an inboard sidewall interconnecting the top and bottom walls, and an open outboard sidewall. A rear frame part is secured to the main frame part and includes a rear frame member extending in the longitudinal direction. The rear frame member has a U-shaped cross-section with an inboard and outboard sidewalls, a bottom wall interconnecting the inboard and outboard sidewalls, and an open top wall. The inboard sidewall at a rearward end portion of the side sill includes a section bulging away from the open outboard sidewall in a lateral direction of the vehicle frame assembly, and the inboard sidewall of the rear frame member is secured to the bulging section of the side sill.