Generally, the formula for
melee power scaling is the following:
Damage = baseDamage * (100 + MeleePower)/100
In your case with 250 MP,
Damage = baseDamage*(100+250)/100
=> Damage = baseDamage*3.5
=> Each hit does 3.5x the base damage
Vampiric bond is similar, except that it grants temp HP. If it had normal scaling, you'd get 20 * 3.5, or 70, temporary HP per proc.
Instead, it scales with 200% melee power, so the formula (as far as I know) becomes the following:
TempHP = baseTempHP * (100 + 2*MeleePower)/100.
Plugging in for baseTempHP and MeleePower...
TempHP = 20 * (100 + 2*250)/100
=> TempHP = 20 * (100 + 500)/100
=> TempHP = 20 * 600/100
=> TempHP = 20*6
=> TempHP = 120
I don't have a Barbarian with access to that skill at the moment, so I can't confirm that this equation is correct or that the scaling is working as intended. If you're getting a different number, please let me know and I'll try to figure out what's happening