The basic rule of thumb when building tailored garments is that you first build the outer fabric. This includes tapers, pockets and any details incorporated into the design.

Then you add the interfacings to the back side of the outer fabric. The idea of the interfacings is that you wear the interfacings. The outer fabric floats over the top with all the stress of wearing the garment taken up by the interfacings.

Then you cover the internal construction with the lining. Linings are fitted quite loose and act not only to cover what is underneath but as an anti-friction layer that allows the garment to drape.

If you look at tailored garments as having three parts (Outer fabric, interfacings, liner) and understand what each does and why it is there it should help you to visualize how they must be done.

The general rule of thumb for the weight of interfacings is use the heaviest you can for the desired use of the garment. Lighter weights for summer garments and heavier for winter garments etc. The interfacing you describe for your vest is very good for a summer weight garment.