recently Kaeko released info on block and block rate:

1VIT = .1% dmg reduction in blocks
1DEX = .1% block rate increase

Pretty strait forward, for example if you allocate 20 points in VIT then you get 2% dmg reduction on every block, on every target regardless of lvl of mob (capped at 75% blocked only happens on mobs 10+ lvl's below your lvl). I would value VIT > DEX since VIT raises your HP and increases DEF in the process and reduces dmg taken.