Instructions
Befriend the maker of the game you're playing. The VIP doors in each game are initially accessible only to the game's creator, but he can also program them to let in specific players he lists in the code.
Look at the description for the Roblox game you're playing. Some game creators prefer not to use individual VIP names, opting instead to program their doors to let in players who are wearing certain game-specific items. If this is the case, the creator will usually post the URL address for the VIP item in the game description.
Copy the URL address from the game description and paste it into your browser's address bar. This will take you to the page where you can buy the VIP item. You will see the item's description as well as how much it costs, either in R$ (the in-game currency) or Tx(Robloxian Tickets.
Click on the payment method you prefer for buying the VIP item. This will bring up a window with the options to either "Buy Now" or "Cancel." Click "Buy Now" to finalize your purchase and start the transaction.
Click on the "My Roblox" link after the transaction completes. Then click on "Character" in the submenu. You will see your new VIP item in your wardrobe. Click its "Wear" button to put it on and you will be a VIP the next time you enter the game.