Against normal enemies, burning hands isn't affected by PV (a 1d3+1 burning hands will do 2-4 damage against a 30 PV stone giant lord). Against fire-vulnerable enemies, burning hands appears to be affected by PV, resulting in messages like: "You scorch the frost giant. The attack doesn't penetrate the defenses of the frost giant." (a 1d3+2 burning hands doing 0 damage to a 19 PV frost giant). Whether or not PV is applied should not depend on whether the monster is vulnerable to fire. In 1.1.1 it seems like PV was applied for non-vulnerable monsters.