Full VIT at 50 is probably the smartest thing you can do. STR works in tiers and you can spend 30 points and still not reach the next tier and if it did push you over you'd have a 1% damage increase which is useless.

When it comes to coil and things the more HP you have to play with the better!

The reason you're loosing aggro to BRD/BLM combo is because -

1. They have excellent AOE damage
2. Foe Requiem (BRD Skill) increases elemental magic damage by 20%. So that BLM's already excellent Damage output is boosted by 20%.

Use things like flash with overpower but don't sacrifice HP for MP with allocated stats.

Cycle through your targets and use a skill on one that you're losing.

You will lose hate with AOE classes. Just gotta accept that and they'll know you won't be able to keep hate forever.

Use stun to your advantage - have you lost hate on a mob but it's pretty close to death? Stun it and move on to the next target. If stun is wearing off before it dies use Holmgang.

Cross class provoke - when you lose a target provoke it and use Tomahawk or enmity combo.

What weapon are you using? Relic?