A microwave is often used to speed up the heating/mixing process. VG and PG (PG to a lesser extent) is quite thick and resists full dilution of flavours at cold temperatures or in pure states. When a liquid heats, it generally becomes more viscous and agitates due to heat or mixes more easily when shaken. There are many people that use hot baths to speed up the mixing process, some use ultrasonic systems that agitate rather than directly heat (friction over induction). There are also comments that suggest oxidation is also a very important part of 'steeping'. This as of yet is an unknown, but would seem more logical with alcohol based products.
My experience is that there are cases that elongated steeps effect juices negatively as well as positively (i lean toward oxidation in this case). I do not use a microwave as it is an unquantifiable measurement for me but, I suggest a microwave is fine at low temperatures or short bursts. If you heat a liquid too much it may well effect the juice. If you do use a microwave for your experiments, please do it with an open container as it could potentially explode if capped and super heated, potentially inflicting a fair bit of damage. I have noted a number of cases where this has happened and a reasonable amount of pain has been experienced as a result.. If your bottle is microwaved, closed and bulging, leave it alone until it cools!
Good luck with your alchemy and please take care..