I found out that 8% lifesteal and 8 vitality damage from haunted steel component does not stack while dual wielding pistols though 25% bonus to vitality dmg stack. Is it bugged or it work as intended?
Working as intended.
The life steal on a weapon is specific to the weapon (unlike life steal on an amulet or devotion).
Whereas % damage increases are treated as global modifiers 
Ok thanks for claryfing this