vUNIT does not actually exist in the pool, which is why it can be thought of as virtual. Instead, vUNIT is expressed as a balance in the pool. For each liquidity pool, the balance is initially zero. However, when Token A is sold, the result is that the Token A balance decreases, and the vUNIT balance increases. The opposite is also true, and when Token A is bought, the inverse is reflected in the vUNIT balance.