Tipping in Las Vegas restaurants is very customary, as it is the main source of income for the waiters. Tipping is usually at 15-20%. Sometimes the tip is already included in the bill - please take a look at the check, it can be called a service charge, or something similar. It is possible to include tips into a total bill if you pay by a card (you can write the tip amount and add it to the total amount due).
Sales tax in Las Vegas is 8.1%. Prices listed on the menu are without the tax, but it will be included in your bill.