Vivid always tries its very best to provide you with the most accurate price available before you place an order. However, sometimes you might experience a slight difference between the price/costs displayed when you place an order and the actual price/costs for which your order was executed, this is called slippage.
You place market orders which are executed with the best price available. Due to the time difference between the order you place and your order being executed, the underlying could go up or down and the actual execution price of the fractional instrument could change. The final execution price will be shared with you in your trade confirmation.