So, if the companies that make the base flavors are smart, the flavors themselves may not be under FDA regulation, only once you add nicotine to them.
However, once you make a flavor and add nic to it, you can't sell it if it hasn't been approved by the FDA. The trick may be to order whatever flavor you want online, then mix the nic in on your own. Problem is, without the brick n mortar businesses to support the companies, it'll cannibalize the industry till only the largest one or two are left.
They can't totally stop China from selling, but if customs stops the package you could be in trouble I believe. Don't quote me on the legalities, but if it's similar to other contraband, that is what normally happens. i.e. trying to ship cigars from cuba to here.