Full VIT is the standard safe-bet but at high levels, 30 points equates to very little of your total VIT/HP pool. But every little bit counts in some cases.

STR will increase your agro generation a bit and your damage output a bit, which actually is useful for DRK since it helps MP and HP regen. IMO this is probably the best choice.

PIE is interesting. I tried swapping over to full PIE to see the results and they are somewhat beneficial. It increased my MP pool a good bit, so it can help with making MP management a little easier and if you wanted to do a big burst with lots of DA usage, which in turn can increase dps a bit as well as eHP from self-heals. I'm not fully certain of its value compared to VIT or STR yet. It seems like it could potentially be more useful than VIT(bonus points from levelling, not gear. You still want tons of +VIT from that and in general), but I still feel that STR will be the best option overall.

So short answer, imo STR.