It can be quite difficult to find plus size vintage dresses for several reasons.
First of all you should note that women in the retro times almost always used girdles or corsets under their dress, therefore they wore very tight dresses and gowns. Even though most of the dresses were very full from the waist down, the waist part was usually very slim.
In some of the retro decades for example in the 40s people didn't have so much money to buy fabrics, because this was still the aftermath of the war. When women got fabrics to make a dress, they used only as much material as it was necessary for the dress and not an inch more.
Still, if you are looking for a plus size vintage prom dress, you shouldn't give up so easy, because it is possible to find them. Plus size vintage prom dresses can be found online for example. Look on eBay or other online stores that offer retro style dresses for a reasonable price.
You should also keep in mind that there are still a lot of vintage reproduction dresses: this means that they are only inspired by the vintage style, but they were made recently. If you find a shop that sells vintage prom dresses inspired by the retro times, you can be sure that they have all kinds of sizes there.
If you find a plus size vintage prom dress online that you really like but you are not sure what the size is and whether it fits you, ask for detailed measurements. Note that in the vintage era especially in the beginning sizing was different than today.
Most dresses ave no labels and it is hard to determine whether they are M, L or XL because they were usually custom made. If you know your measurement you can avoid any bad suprises and enjoy the dress fully.