Pretty sure 7-9k is not "optimal." Pretty sure zero vitality is "optimal." That's 4,712 health, allowing Diamond Skin to take up to 13 full hits.
Your right. The less vitality the better, though its pretty rare to get a full set of gear with zero vitality, so I was basing it off of what I have at the time. But yes, zero vitality is optimal.
Sure, until you run over a fire trap