Since there is no tax charged on the purchase of the current dining plan (at $35/adult, increasing to $37.99 on January 1), there is no reason to expect there to be tax charged on the dining plan when purchased through
DVC. In other words, the $37.99 is not taxable (which makes sense, since this would constitute paying tax on a price that already includes a portion allocated to sales tax as well as a portion allocated to the tip).
Ralph