Your available funds are determined by your employer and the plan they selected. For example, my 401k is through Fidelity, but they offer the Vanguard Target Retirement funds as an option. VTSAX is not included, but Fidelity has their own index funds you can select. They're easy to find, they're the ones with the really low expense ratios!
You may not be able to select VTSAX directly, but you can possibly find something very similar, depending on your plan. If there isn't a total stock index, you can either go fully large cap, or recreate VTSAX with approximately 70% large cap, 20% mid cap, and 10% small cap.