VIT is officially the controlling stat for tanking, however, having said that STR is still very useful and well worth investing at least some of your AP into, as like others have said it increases the damage you deal and thus allows you to hold hate better (although with GLA and later PLD as long as you use Flash effectively in combination with the right weapon skills and Provoke in an emergency you shouldn't have too much trouble keeping hate on your in most cases even if you go for full VIT).

The best advice I can give is to experiment and find what works best for you, full VIT, full STR or a bit of both. As the Keeper's Hymn item that allows you to reset a class's AP is now very cheap to buy it's definitely possible to experiment with how you spend your AP rather than just 'chuck everything into the class's main stat'.