I'm fairly certain it would work similar to cigarettes, the average clerk and customer won't even be aware of it. It will be added into the wholesale cost to the retailer. Now... how that works for those "manufacturing" it, I'm not sure. Possibly they'll just pay the tax on the wholesale bulk nic liquid? If they do, we shouldn't have to pay a second tax.
ETA: In which case local "house brews" would become the more economical choice. That is, if shops are still allowed to manufacture house brews...