Please don’t try to bleach the dress! If it is from the 1940s it is a very early example of polyester (invented 1941). Why has it yellowed? This is not a stain, which might be washed out, it is, as you say, the result of aging. The polyester may be breaking down and becoming increasingly fragile, and the use of chemicals such as acetic acid (vinegar) and peroxides or chlorides) may accelerate its destruction. This is an opportunity to educate yoiur visitors and celebrate the fact that you have such an early example of this manmade fibre. You could explain why it now looks different and remind people of the use of bleaches and optical whiteners – which have distorted our expectations of whiteness.
Hope this helps.