Defensive Duelist

If one has this feat you get Parry ability

 If you’re holding a Finesse weapon and another creature hits you with a melee attack, you can take a Reaction to add your Proficiency Bonus to your Armor Class...

 

If you have a dancing Dancing Rapier, (the weapon begins to hover, flies up to 30 feet, and attacks one creature of your choice within 5 feet of itself. The weapon uses your attack roll and adds your ability modifier to damage rolls.)

While hovering and attacking do you still get the Parry AC bonus?  Does it matter if the Rapier is or is not attacking the opponent you are reacting to?

If the weapon is hovering and attacking on its own, you're not holding it. So no.

Quote from Character77006 >>

 

While hovering and attacking do you still get the Parry AC bonus?  Does it matter if the Rapier is or is not attacking the opponent you are reacting to?

Natrel is correct, but just to answer your second question: no, it doesn't matter, and it's not really possible for the Dancing Rapier to be attacking something at the same time as something is attacking you.

 If you’re holding a Finesse weapon 

RAW you have to hold it in the moment your character is being hit, so it would not work while the weapon is floating around I think.

You have to parry with the weapon in your hand. The magic of the Dancing Rapier allows it to attack not to parry. 

Nothing prevents you from equipping another finesse weapon while the rapier is doing its thing though.