If you are trying to achieve a faster steeping, there are other ways. A simple way is to make your own agitator. Duct tape the bottle to a reciprocating or jigsaw, shake well for 10 minutes. Seriously. It works.
If you are set on a heat steep, consider making the liquid and then putting the finished product into a hot water bath in a slow cooker. There are several threads on this and other methods (like using ultrasonic cleaners and such) over in the DIY liquid section
DIY E-Liquid
As itskohler said, you really want to be steeping the finished e-liquid together with the flavorings for best results.