What sort of breakpoint are you hitting with Vehicle Weapons?
I'm not sure if "breakpoint" is a common starcraft term.
I know it from Diablo 2.
In Diablo if you stack increased attack speed your character attacks faster.
Eventually it comes down to precise frame counts.
+30% IAS might give you zero boost over +25% because you take the same number of frames to make an attack, but +31% might give you a 10% boost over +30 because you attack 1 frame sooner.
This is a breakpoint - your upgrade doesn't pay off in the % it looks like because you have to hit some threshold to get the benefit.
We see this really obviously when Zealots are 1 attack ahead of Zergling armor - instead of dealing 10% more damage, they kill in 2 hits instead of 3 and effectively deal 50% more.
The same thing can happen in reverse.
If Zergs are ahead on carapace this might help hydra or whatever.
But zerglings vs zealots will still die in 3 hits so the Zerg gets ZERO benefit there vs being even on upgrades.
As I said, pros are often neglecting +2 Vehicle Weapons these days.
Vehicle Weapons has some funny breakpoints.
Unless I'm doing my math wrong, it always takes 3 direct siege tank shots to kill a zealot, goon or reaver.
I think +1 makes Dragoons die in 2.5 shots (so 50% splash can count as the third shot).
I admit I'm skeptical whether +1 is even really good.
My guess is that pro Terrans are figuring out (or losing, which is a slow way to figure out) that +2 weapons doesn't really hit great breakpoints in terms of actually killing Protoss. If so, +2 can actually make your army worse by increasing the splash your own units take (although it does also buff vultures and goliath ground attack).
I checked and (assuming I did my math right) +1 should let Goliaths 5-shot interceptors (which +2 also does) unless the interceptors get +1 air armor or +2 shields
On the other hand
Vehicle armor has no splash downside and as far as I can tell always shifts breakpoints favorably vs zealot, goon & interceptor attacks.
So perhaps TvP you stack armor rather than attack, and only get +attack if there are specific useful breakpoints ?