30 STR should be around a 7% increase in damage output and probably similar but marginally reduced results for threat. I don't know how Flash hate is calculated off the top of my head. Defensive gains from block and parry tiers, should you reach one via allocating STR, are minimal.

Simply explaining the net increase in HP and output from VIT and STR doesn't really do the question justice though. More STR is pretty much always useful from an efficiency standpoint. VIT not so much; it's pretty easy to figure when you need more HP vs when you don't. Exactly how much more is a bit fuzzier, but unless you're building to survive a specific hit or chain of hits where you know exactly how much damage you'll take and you'll be at max HP for it every time then it's difficult to construct a solid argument for putting some amount of points into VIT that is neither 0 nor 30. Also, don't forget that you can gearswap between fights - STR accessories for fodder and VIT for stuff where you're concerned with survivability is a strong option.