Basically what's already been said. Go 30 vit unless:

There are values referred to as "break points". Meaning you get a % increase to parry, block, dodge, etc. If for example you're at 339 str and there is a breakpoint at 340 str to get another 1% to parry I would think its worth it to go 29/1 vit/str for that 1%. I don't know the break points off the top of my head, although I probably should, but you can probably find them easily enough.

That's basically the only time you wouldn't want to go 30 vit.