The more vets you have, the more recruits you can take off when they cap.

As soon as the gunner reaches the Reload /Acc cap, it doesen´t get any better, the vets do help on lower level, so more fun with lower tier ships, but at lvl 120 base and vets don´t do anything. You can run into problems with UK Acc Gunner, they Reload Cap very late.

but in WaW (hardcore mode) it is nice to have as much vets as u can have. U will fire with main guns as in CL/CA. Specially KMs or IJNs gunners. UK have a bit longer reload.

Yes, I got that. At which lvl do ijn gunners reach cap?

2.7 mil TA for reload
1,9~2,1mil  TA @ level 120

That is the shortest answer.

I have 214 vets on each of my usn gunners (not elite)
It made a difference before Iowa. But at 116 on Monty, I don't see much of a difference between my accuracy  and any other Monty. 
H44 gunners, same thing. 
Spend your experts to vets on your engineers.
I went for at least 200 on every engineer. You get the speed cap much faster. And length of OH makes a huge difference in game play. 

The number of vets will only define how early more vets/experts become useless. Once you hit around 2m TA in accuracy and the 2.7m TA in reload then additional vets/experts will only increase SD and repair by very little.

Youd benefit more by putting those vets on supports/pilots/AA etc.