Does that mean that you get 20AP for each kill in single VATS session or 20AP for kill in a single VATS session no matter the number of kills?

20AP for kill in a single VATS session <-- should be that.

Oh, I won't take it then..

Keep in mind you could simply target one foe at a time in VATS.

It was amazing in Fallout 3. Made battles almost no fun.

Mhmm. VATS-up 3 targets... kill all 3... have full AP bar, queue up 3 more VATS targets 1 second later, kill all 3, still have full AP bar... look around for more targets... nothing but dead bodies.

Grim Reaper's Sprint:

Terrible, silly perk IMO; The result of a contest ~against some 17,000 other entries (if memory serves).

*Granted many of those were likely variants of each other, but still...