You are warrior so it is correct your health to decease with 90 when you remove the item with +4 vitality bonus because +4 vitality gives 90 helth to a warrior class heroes.
4*1.5 = 6 vitality and 6 * 1.5 * 10 = 90 health
There my be a little misunderstanding here I think.........
I was on 450 Health with the weapon WITHOUT the +4 vitality............ Removing this weapon reduced my health by 90 (450 to 360).........
On fitting the weapon WITH +4 vitality, my health increased by 60 (360 to 420)............ According to your formula, this should be 90 ??
On fitting the original weapon again, my health decreased by 60 (420 to 360)...........
Hope this clarifies things.