Shoes made of synthetic materials do not stretch as easily as leather shoes, but it is possible to stretch them. You will have to combine a chemical treatment along with some pressure applied from inside the shoes. Pick up the shoe stretching chemical either online, from a local shoe store, or from a cobbler. Make sure you get the shoe stretch chemical labelled for vinyl, because the one specifically for leather will usually not work on vinyl.
Cover the work surface with a layer of newspaper or a cardboard. This will protect your tabletop or counter from shoe stretching liquid over-spray.
Shake the shoe stretching liquid, and hold the bottle upright, about seven inches away from the shoes. Pick one shoe to do first. Spray an even layer of liquid onto the vinyl, or non-leather, surface of the shoe you picked to stretch first. Stop spraying before the liquid begins to run, but spray a layer heavy enough that the shoe is visibly wet.
Slide the shoe stretcher tool all the way into the wet shoe. Turn the handle of the shoe stretcher counter-clockwise to expand the stretcher to the desired size.
Spray shoe stretching liquid on the second shoe and insert the second shoe stretcher. Expand the second shoe stretcher, then let the shoes dry completely. Allow the stretchers to remain in the shoes until you are ready to wear them.