I'd definitely recommend against using a keepers hymn to trade for vitality as a DPS.

VIT accessories give 13 VIT
STR/DEX/INT accessories give 11 STR/DEX/INT

You get more bang for your buck swapping in vitality accessories than changing your main attribute points. You can get full det/crit/VIT accessories so you don't have to sacrifice secondary stats.

The only reason to use a keepers hymn is if you've already swapped all your accessories for VIT and require more. I do this on my healer, but I think it's definitely not worth it as a DPS outside of maybe a BLM (where a lot of your moves are crowd control and do next to 0 damage, so it's less of a loss).

For PvP skills, 3 points into VIT = 12 VIT = ~200 hp. Allocating here is a personal preference and depends on your job. If you have 3 points to spare, you could instead put these into STR and get 12 STR. As mentioned above the STR/VIT exchange rate of accessories is 11:13, so it's only worth putting these PvP points into STR if you've maxed out the accessories and need more HP, otherwise you're better off spending them in STR.

As Zeopower6 mentioned food is huge. Food = ~200 hp + secondary stats and it lasts for 40 mins if you have the FC buff. You don't have to trade anything for this hp boost, so it's definitely worth it.

Another way to increase your hp is make sure there's a tank in your group. They give a group +3% VIT buff, which is also roughly another ~200 hp.