This is Thursday, according to this site, Monday and Tuesday was 100 degrees. So if heat index it was 120 degrees - I would guess that so many cows have little or no shelter, and if they did not have plenty of water then I suppose that maybe it might be conceivable. In normal times I guess that I could believe it, but these days who knows?
I raised cattle.
120 with heat index every year, little to no shade, sometimes we forgot to water them, no pond to wade in, and not one died.
I don't believe these died from heat stroke.
As for vaccines, cattle don't get them every year, like dogs and cats, but only when they are young. So a vaccine doing this is unlikely.
Poisoning or 5G would be my bet.