This is bonus damage, and two bonuses from the same named gamed element don't stack. More importantly though, there is a little known clause in the first adventurer's vault that says you can't use the second vanguard weapon on the charge:
Many weapons have properties that provide a constant benefit. To gain the benefit of a weapon’s property, you must be wielding the weapon. Unless specified otherwise, a property affects only the weapon to which it’s attached. For example, a +2 cunning dagger, which bestows a –2 penalty to an enemy’s saving throws against your weapon powers, affects only powers that are delivered using that weapon. You couldn’t hold the weapon in your off-hand and gain the benefit of the property on powers delivered using a main weapon. (AV 56, Emphasis mine)
I've expounded a lot more on this particular sentence in this answer. But that's neither here nor there really, the two wouldn't stack even without that as they are the same game identified source.