Gandalf's staff is made of wood and should be sliced to matchsticks with Vader's lightsaber. Yes, Gandalf can make a spherical energy shield that can withstand a Balrog's whip and sword, but can he concentrate it to the staff itself? I doubt it. Glamdring, was designed to fight orcs, not Sith. Again, sliced to tiny metal shards like Narsil. Again, no, the shield couldn't be just the size of the sword.
Next up, force lighting. How is Gandalf capable of creating force lightning that can counter Vader's? From what I understood of The Grey's powers, he can summon fireballs and lightning from the sky, not his hands.
Last, shapeshifting. Gandalf's technique wasn't that of trickery, nor would his powers allow him to do that while being The Grey. It's entirely possible he could if he were The White. However, to be The White means that a person is good and defends good, therefore trickery is a no-go.
Vader would've tossed some force lightning at Gandalf's sphere of protection. Gandalf would counter by setting Vader on fucking fire. End of fight.
Compared to the power of a wizard of Middle Earth, the force is insignificant.