If you look upon the explanations what's hidden from in-game definitions, you'll see that certain mods reduce AP cost by percentage. For Ballistic Pistols and Rifles, the AP cost modification from turning semi weapon into automatic one is -50%. Legendary 25% mod stacks with this directly, putting the reduction at 75%. You need 25% more AP reduction from other mods, and this you can achieve by using:
Adding up the percentages from the modifications and from legendary effect, we get 105% AP reduction. Does this mean that we are now using 0 AP when shooting with VATS? If this were the case, the subtitle of this article would not be: "... which once were completely reducing AP consumption in VATS".
Thanks to Eckserah, founder of Fallout 76 Dataming, i found out that with the Wastelanders going live, the AP reduction counting has changed. Instead of adding up percentages, we now multiply the biggest reduction (automatic receiver, -50%) by next reductions, which gives us:
.50 * 1.25 * 1.05 * 1.05 * 1.05 * 1.15 = 0.83204296875
So, if you configure your weapon to become a zeroVATS weapon, you will reduce your VATS action points consumption by 83%, and hence you will be capable to achieve the highest sustained DPS.