Why don't modern F-16s incorporate the VISTA thrust vectoring?
Thrust vectoring adds weight, cost and complexity, and the benefit isn't important enough to outweigh that.

Various aircraft have been used to experiment with thrust vectoring, but the list of thrust vectoring fighters in service is fairly short:


F-22
Su-30 (newer variants)
Su-35S
J-20 (fairly new)
Su-57 (very new)


The F-16 was envisioned to be a small, light, and cheap fighter. It has certainly grown heavier and more expensive over the years, but that is still one of the big selling points. Although it worked fine for the VISTA experiments, the limited space, power and cooling on the F-16 is best used for other upgrades, such as better sensors. And the F-16 is still selling fairly well. Lockheed is still getting enough orders that they moved the production line to make room for the F-35 rather than shutting it down.

Even the F-35A, intended to replace the F-16, adds stealth but not thrust vectoring. If you can kill your adversary before they even know you're there, dogfighting ability becomes less important. There's also the reality that US manufacturers build primarily for the US, and for US interests. There might be resistance to exporting an even more maneuverable F-16 when it may already be close in capability to the F-35.