First, vinyl is a superior insulator because heat doesn’t efficiently transfer through it. Think about a hot pan on the stove. Only the bottom of the pan is touching the stove, but the heat is transferring through the metal and into the handle. With a nice vinyl handle on the pan, you can hold it by the handle while stirring your hot food.
The same principle applies to vinyl replacement window frames. The sun heats up the exterior side of the frame, but the interior side stays cool. No heat transfer.
Second, the glass unit is a unit of two panes of glass, separated by an insulating spacer. The exterior pane gets hot, but because of the air gap between the panes and the vinyl or butyl rubber-based spacer, the interior pane is insulated from the exterior pane. No heat transfer taking place.
Finally, another element of the IGU is the combination of low-e coatings and gas. Together, they block UV rays from entering the room. This means that not only do your new vinyl replacement windows reflect heat back off the window surface, but they also block carpet-damaging and fabric-fading UV rays.
Now you can stand in front of your windows without sunglasses and enjoy the view without getting overheated. Your air conditioner won’t always be able to compensate for all that radiant heat.