I am guessing you are mixing just menthol crystals and VG, no nic, flavoring pg etc.
I can say from experience that mine do melt very fast by themselves with a bit of swirling in a nic based
juice, a bit longer if I just let them sit and do not swirl it around. (a few mins and it is done)
But 5 seconds (or so depending on the power) in a microwave is one method, as well as sitting the bottle in hot water for a little while in the event your crystals are not dissolving in your liquid.
Another possibility would be to put them in an empty bottle, add a little alcohol (vodka or some other clear type preferably) to disolve them, then add your vg etc. If you only are using a few crystals then just a few drops of alcohol should do the job here.
I dont know if they will melt in plain vg over time or not as I never tried, all my
juice has nic in it which I am fairly sure assists in disolving the crystals.