With statements like that you just successfully banned yourself from ever being adopted by real life merchants ever. Merchants can already escape the VAT via cash yet most don't do this - why? Because the risk of dong so is in no relation to the reward of doing so. No merchant in their right mind would ever accept a coin whose customer would annoy him with the request of "selling them the stuff under the table" especially if accepting the coin would basically guarantee him he instant attention of the tax authorities.
VAT cannot be avoided by taking cash unless you are implying that merchants can avoid tax completely by not declaring the transaction (this is illegal).
All transactions that a business processes must be declared to the relevant tax authority for their country otherwise the business owner may face jail/prison time (FIAT or Crypto).
Merchants have been getting harassed by customers since the FIAT system was born. Customers ask for discounts and expect better treatment if they are paying in cash and not debit/credit/cheques.
Most of our merchants however will carry over the saving to the customer giving them an incentive to purchase goods in Sterlingcoin and Bitcoin i.e. 10% off all items.