Why was Vision so powerless in Avengers: Infinity War even though he possessed an Infinity Stone?

I don’t know. Viz and Witch should have had no trouble with Proxima and the other one. It made no sense that they needed to be rescued by Cap, Widow and Falcon. Great scene, though.

If you will recall Corvus Glave skewered him with the weapon he had (a glave), which rendered him unable to phase and weakened him quite a bit.

Vision was suffering from being run through with a spear.

Mind stone gives users telepathic abilities: remote access to minds (telepathic conversations) and mind control. Vision never exhibits either ability, so I don't think he knows how to use it. He is a synthetic life form, not an organic being, that might have something to do with it. Mind stone probably does not regard him as an user, more a container.

But even he does know how to and can use it, mind stone does not have much destructive power.