He doesn't have a money vault. He has almost zero cash holdings. If you tax him, then he will have to sell assets to pay for those taxes, which removes money from the economy as that money was being actively spent on his assets. The only situation when money supply changes is if someone has a literal mattress full of cash hidden away. Even if you just have money in a savings account, it acts as a fractional reserve for the banking system meaning that there is a net decrease in money supply when you give it to the government as banks can no longer use it as the backing for loans.