The Item power as written does not work.
I double checked the online compendium as I too shared dopplegreener's suspicions that it might instead be an immediate interrupt, but you have correctly copied the power down.
Parrying Weapon Level 2+ Uncommon
A wielder of this weapon never truly lowers his or her defenses.
Lvl 2 +1 520 gp Lvl 17 +4 65,000 gp
Lvl 7 +2 2,600 gp Lvl 22 +5 325,000 gp
Lvl 12 +3 13,000 gp Lvl 27 +6 1,625,000 gp
Weapon: Any melee
Enhancement Bonus: attack rolls and damage rolls
Critical: +1d6 damage per plus
Power Daily (Immediate Reaction)
Use this power when an enemy makes a melee attack against you. Make a melee basic attack against that enemy, with a power bonus on your attack roll equal to this weapon's enhancement bonus; if your result exceeds that of the attack roll against you, the enemy's attack misses. The melee basic attack you make to block your enemy's attack has no other effect and does not deal damage. -Published in Adventurer's Vault, page(s) 74.
The reason this doesn't function is because Immediate Reactions only occur after the triggering action has completed, meaning the triggering attack would already be finished and you'd already have been hit.
Reactions: An immediate reaction lets a creature act in response to a trigger. The triggering action or event occurs and is completely resolved before the reaction takes place. -Published in Player's Handbook, page(s) 268, Rules Compendium, page(s) 195.
Whereas Immediate Interrupts occur before the triggering action finishes and then the triggering action resumes (and completes if it is still able to).
Interrupts: An immediate interrupt jumps in when its trigger occurs, taking place before the trigger finishes. If an interrupt invalidates a triggering action, the triggering action is lost. -Published in Player's Handbook, page(s) 268, Rules Compendium, page(s) 195.
The Item power was most certainly meant to be an interrupt power. As an interrupt, Parrying Weapon's ability could leap in at any point during the triggering attack, including directly after the attack roll (so that you could wait to see if it was going to hit).
The Adventurer's vault has other vaguely worded items and sections. This question of my own, and others in the past have resulted from poor wording in the adverturer's vault.
Because RAW the item power would do absolutely nothing I would suggest that you/the DM house rule the item to be have a daily interrupt power vs. a reaction power.