Personally, I don't think there are many situations where an extra 1k+ HP would make a real difference in A5S - A7S if you have been farming it for a while and have sufficient HP from i240 left and right sides. So, I think wearing 1 piece of slaying for the higher stat weighting is probably okay. I think 2 pieces starts to get a bit iffy as 2k+ HP does actually start to approach certain thresholds. I think there is very little reason to wear 3 slaying pieces -- I don't even think there are 3 slaying pieces with decidedly better stat weights.

During prog and before you out-gear content, I think you obviously wear VIT gear.