Quoting from the web page linked below: "Unfortunately, plastic chrome plating is not as simple as applying a coat of chrome to the part. Typically, we will first coat the part with a layer of copper or nickel through a process called electro-less plating. This means that we do not use an electric current to apply the plating. Applying this coating makes the part conducive, which makes it possible to apply the chrome plating later on."
Whole process here (this is NOT vacuum metalizing): https://www.vacuum-metalizing.com/plastic-chrome-plating/
EDIT: Electro-less nickel plating, including on non-conductive materials... https://hcsplating.com/resources/nickel-plating-guide/electroless-vs-electrolytic-nickel-plating/
EDIT 2: After the electro-less plating is applied, either copper or nickel (apparently copper in Trumpeter's case), plastic parts can be electroplated with actual chrome in the usual way.