Is a 20% forced transition in xfoil similar to adding a vg?
No

It is more like putting a turbulator (such as a trip wire) there. Vortex generators will create a small vortex (hence the name) which puts much more energy into the boundary layer. XFOIL has no mechanism for simulating vortex generators.

Adding a forced turbulent transition will weaken the boundary layer downstream of its natural transition point, so it will tolerate less pressure rise. I assume you put them on the upper side: Now the airfoil will stall a bit earlier (at a lower angle of attack and lift coefficient).

15-20% is an unusual location for a turbulator, especially on the upper side. Their best location depends on the Reynolds number and the pressure distribution. On gliders, they make most sense on the lower side at around 80% of chord. On the upper side they might help a bit in part of the polar when placed at around 50%.