How should I repair mounting holes drilled into pvc window frames?
White silicon may work to plug the holes but if you use it make sure to get the surface finish the way you want it before the stuff dries. It is not a material that will sand very well after the fact.

You may also want to try a bathroom tub and sink sealer product. These products bridge gaps fairly well and dry staying a nice white color. The product that I have used for sinks and tubs is the one pictured below.

Also be aware of several things. If screws had been applied into the PVC window frame the screw threads probably made a portion of the PVC surface stick out at the hole edge. This may need to be trimmed carefully so that your patched holes will be flat. If the window frame was made only of PVC with no internal wood structure the only thing the patch material will be adhering to is the edges of the hole. This will mean that the patch could pop in or out of someone were to be poking at it.