A vehicle door assembly comprises an inner panel integrally formed by upper and lower horizontal beams interconnected by front and rear upright beams, and an outer panel bonded to the inner panel. The frame portion comprises opposite front and rear U-shaped side frame members non-detachably attached to each other by upper and lower intermediate frame members so as to form a continuous perimeter. The side frame members are made of a first steel material and the intermediate frame members made of a second steel material, which is different from the first steel material. A method for manufacturing the inner panel comprises the steps: providing U-shaped sheets of the first steel material, providing top and bottom sheets of the second steel material, non-detachably connecting the U-shaped sheets to the top and bottom sheets so as to form a blank, and stamping the blank into the frame portion.