Difference between virtual and ground plane
First off your Instrumentation Amp resistors should be high precision types, 1% or better.  Without this your DRL circuit, as connected, is amplifying the gain error and DC offsets.  (As a side note, the DRL circuit and connection do not quite look correct.)  If this circuit must be used in this form with non-precision resistors consider changing one of the 1.8k parts to a 1.5k in series with a 500 ohm potentiometer, then tune to zero with shorted input signals.

Also, your input signals label V+ and V- are confusing while you discuss power supply voltages of V+ and V-, (obviously your input signals would be better named Left, Right, for biomed connections).

As other are hinting, the vref should be used to create the virtual ground with other input, see info below.

Lastly, for more specific info see the TI app sheet/report SBAA188-July 2011, on CM Rejection using Right=Leg-Drive Amp, at:
http://www.ti.com/lit/an/sbaa188/sbaa188.pdf