I'm an avid shooter. Have been since I was a kid, and this sort of thing comes up in the gun world all the time. Every time a liberal politician is elected, people stock up, thinking that soon they won't be able to get the things they enjoy.
So for those who don't know where to begin, let me tell you, from a guy who is an old hand at stockpiling.
You have to look at the items that will wear out, run out or break. I'm not talking about mods or switches, electrical switches, batteries, wire, chargers, solder and battery boxes will always be available and you can build a mod out of a radio shack battery box for a few bucks. 10 or 12 spare 510 connectors should set you up for a good long while if the sale of them becomes completely illegal.
Atomizers and cartomizers are the biggest wear part. Luckily, they're cheap. If you're worried, but a few extra every time you order and put them in a box somewhere out of the way, for a rainy day. That's not a bad practice anyway. If you lose your job or get too sick to work, its nice to know you don't have to worry about spending the money on habits. If you buy in bulk from the Chinese suppliers, you can get them really cheap. Another option is to get set up with one of the rebuildable atomizers and learn to roll your own.
Liquid Nicotine is the other weak spot. It really has no other use, so could be easily targeted by bans or high taxes. The good news is that its also relatively inexpensive, and if you watch some of the suppliers, they have 50% off sales all the time. Keep it in a cool dark place and it should last a few years. Flavoring, PG and VG won't be hard to come by, they're used for other things so will always be available.
The mantra of the prepper has always been 'Buy it cheap, stack it deep.' If you're worried about a ban, its a good way to think. Personally, I'm not worried. I've got probably a year's worth of atomizers and six months of juice. But they won't be banned. Taxed, certainly, but not banned.