The Venue 11 7130/7139/7140 BIOS (Basic Input/Output System) supports pre-boot BitLocker pin authentication and does provide a virtual-touch keyboard for PIN input.
For the best results, you should have the latest version of the BIOS available for your Tablet installed. For more information refer to Dell Knowledge Base article Dell BIOS Updates.
In order for the pre-boot virtual keyboard to be active on startup: