That's because VIT controls your HP level and physical defense, it does not effect your hate values - the only stats that would have any effect on hate would probably be STR and DEX (physical attack damage and attack accuracy respectively). But yes, VIT is the most important stat for GLA/PLD - to hold hate you need to use your abilities effectively and make sure your gear (sword, shield and armour) are up to date. As Eightbit mentioned, Shield Lob, Flash and your weapon skill combos are the most important ways of keeping hate on you, with Provoke a back up in case you lose hate during the fight.