Disabling MBP display with external monitor connected in Clamshell Mode - Works with DP->DVI, but not DP->DP
As a start I think we need to establish if you can get your display to work correctly while your MacBook Pro is in closed-clamshell mode. To do this, follow these steps:


Connect a USB keyboard and mouse to your MBP (or pair a Bluetooth keyboard and mouse with your MBP)
Connect your MBP to an AC power source
Connect your Viewsonic display to your MBP 
After your MBP desktop appears on your Viewsonic display, close your MBP's lid 
If your display goes to sleep, press a key or click your mouse to wake it up


NOTE: If your Viewsonic display doesn't wake up, then disconnect the display and shut down your MBP. Now connect your display while your MBP is off and then switch the MBP on. After logging in close the lid. Remember to keep the external keyboard and mouse connected