Sunlight is definitely the thing that will make your juice unusable the fastest. Air is number 2, followed by heat.
Heat does break down nicotine pretty quickly once you go over a certain temperature to my understanding.
As far as glass vs. plastic, glass will absolutely keep a liquid better over time than plastic. If you are talking about a couple years, plastic might be OK, but longer should be glass.
And there are a few types of plastic out there, the crystal clear PET plastic is not what you want for long term storage (or any storage really ). You want LDPE or HDPE, Low and High Density Polyethylene, cloudy or solid opaque plastic for any kind of storage up to a few years. PET breaks down and releases gasses into the bottle.
