Hello,
When you want to create a new price by quantity in a product, the drop-down list of taxes does not work.
you have to back up first with a dummy tariff and a range of quantity, then return to change fees apply.
It is not a normal behavour.
( tested in two differents sites with j3.4.5 and vm 3.0.10)
thanks, Serge
Normal VM procedure is to hit the add new price button, enter the base price and hit save. This generates the "new" product price entry in the db and you can then set tax etc.
It has been this way of working since quantity pricing was introduced.