K.S.
I would NEVER EVER threaten him with the vacuum cleaner if he's not doing something you or your husband want him to do. All that does is reinforces his fear. I'd seriously consider bipping my husband upside the head with the vacuum cleaner if he did something that cruel!
When I get ready to vacuum, I call out "Big Noise, Big Noise" so my boys don't get startled by the noise. I would also start the vacuum cleaner as far away from the kids as possible - so they wouldn't have to deal with a sudden loud noise right in their ears. My oldest went through a very brief period where he'd shriek about the vacuum - I think he was around two or three. I would let him know that he could wait in his room (and there wouldn't be anyone in there with him) and would just calmly go about my vacuuming. I didn't do anything that he could consider chasing him or following him. I just slowly and calmly vacuumed the floor. As soon as I was done, I'd say in a very bright voice, "All Done!"
It sounds like you're catering to his fears by babying him through something that has never and would never hurt him. It also sounds as though your husband is encouraging his fear by tormenting him with threats of the vacuum cleaner. (He's a lawyer, right? I'd tell him lawyers are supposed to be logical and this behavior is NOT logical.)
To get him over his fear, I'd leave the vacuum without with plugging it in and running it until he stops freaking out about it. Then, I'd make a point of pushing the vacuum around the house without turning it on. (I'd probably even "decorate" the vacuum by putting some ribbon and clothes and stuff on it - to make it look funny.) I would not say a single word to your son about the vacuum except to tell him what you're doing. ("I'm just going to push this silly looking vacuum cleaner around the living room - but the vacuum has a headache so he doesn't want to be loud today.") When you decide to turn the vacuum on, I'd give him advance notice in a very calm way ("Mommy is going to vacuum the living room and it's going to be loud. If the sound hurts your ears, you can go wait in your bedroom and I'll come and get you as soon as I'm done") but I wouldn't coax or argue. I'd just tell him what I was going to do. Then, I'd let him go to his room alone and would vacuum in a spot that is as far away from his room as possible and I would only vacuum a very small area. Then I would go to him and tell him that you knew he was a big brave boy and he did it! He's going to cry and get upset, but if he's a smart little guy he'll figure out very quickly that the vacuum isn't going to hurt him.