The on-board VTG source is set from Atmel Studio. The VTARGET jumper must be mounted to use this source. The on-board supply can be adjusted from 0.9 to 5.5V in steps of 0.1V.
VTG can also be set to 0V, but due to hardware limitations, the on-board generated VTG cannot be set between 0 and 0.9V.
See the help section for the Atmel Studio Programming Dialog for information on how to adjust VTARGET.