What you are making is what is called a "saturated solution." That is, you dissolve crystals in your solvent (PG, Vodka, whatever) until no more will dissolve.
This can cause a bit of a problem in some cases, because the degree to which menthol crystals are soluble in a given solvent varies with temperature. So with winter coming upon us the amount of menthol that a solvent could hold at 70 degrees may be more than the same solvent can hold at 50 degrees. As a result, menthol may crystallize from the solution at lower temperatures.
If, however, you plan to keep the solutions in the same area where you made them rather than a garage &c. this shouldn't be a problem. It could, however, cause an issue in some cases with PVs left in a car overnight etc.