I found this about the MN excise tax.
E-cigarettes Mail order purchases from out of state are subject to the tax but in most states that's true of all out of state mail order purchases. Zero nic liquids are NOT taxed so buying zero nic and adding nic at home would be a work around. Vaping hardware is not taxed so long as it comes with no fluids containing nicotine.
If my state, Michigan, did the same as MN I would make my own e liquids. I'm seriously considering that already because of the possibility of taxes. Currently I'm buying liquids online in 120ml bottles. Vape shop prices are already too high for me.
I suspect the devices aren't taxed because of legal barriers.
The customers who came in to the vape shop while I was there were young and old and obviously on limited budgets. They want to avoid the harm of cigarettes and they need the money they save by vaping. Taxing their smoking or vaping is cruel. Dig down and the people who run things in america are a nasty bunch.