All the larger wing pylon mounted engines, being close to the wing due to nacelle size and ground clearance considerations, shed vortices which cause undesirable flow problems, particularly in the stall regime.
The VG which you a looking at (often referred to as a nacelle chine but a VG, nonetheless) generates a strong vortex at high alpha which reduces the problem of the nacelle shed vortex and reduces the stall speed a few knots.
Have a look at the sticky URL thread - there is a page there which describes the situation very clearly.
regards,
JT