Do the L1011 tankers have pods, or just a central fuelling station at the fuselage ?
If they have pods installed to the outer wing, these might disturbe airflow there, so the VG had to be installed to counteract the effects. On other tanker conversions there were also some vortex generators added to the outer wing to minimise pitch effects occuring at high speed with the pods installed.
Does the 1011 have a supercritical wing ?