A vehicle door assembly includes inner and outer door panels, first and second support brackets, a reinforcing panel, a window opening support panel and a side mirror assembly. The first support bracket is attached to the inner door panel. The reinforcing panel is attached to the first support bracket. The second support panel attached to the first support panel and the reinforcing panel. The window opening support panel is attached to the inner door panel, the second support panel and the reinforcing panel. The outer door panel is fixed to the inner door panel such that the inner door panel and the outer door panel define a door cavity therebetween. The side mirror assembly is installed along an exterior side of the outer door panel. The side mirror assembly includes fasteners that extend through the window opening support panel and the second support panel.