DnD4 if full of seemingly conflicting rules.
Your speed decreeses if you wear heavy armor.
Your speed does not change in heavy armor if you are a Dwarf.
You use Strength for Melee Basic Attacks.
You can use Dexterity for Melee Basic Attacks if you are an Executioner.
There is no problem however, the rulebooks tell us how to resolve this; if they are contradicting, the one with the narrower scope wins, otherwise both are valid.
So I do not think anything is ambigious here.
To use an Implement it is enough to hold it, to benefit from an Armor it is enough to wear it. (Rules Compendium)
The properties of a Weapon apply to itself only. A weapon can not have Darkvision, so the property of a Midnight Blade obviously applies to you. On the other hand, an attack can be connected to a weapon, so you have to actually attack with it to benefit from the property of a Vanguard Weapon. (Adventurer's Vault)
Both rules are true at the same time, the RC for all magic items, the AV for magic Weapons only. The second is the smaller subset, so it overwrites the first.
Look at it from the other side, what whas the aim of the AV rule, if not what I explained above?
Consider the Staff of Ruin. It was introduced in the same book, and originally it gave an item bonus whether you used it to for the attack or not. It was errataed to limit the bonus to attack made with it. Vanguard weapon was never errataed, because the bonus did not apply to other attacks in the first place.