I have used a shop vac on my horses for years. I just love it and so do they, once they get used to it. Horse vacs are loud too and cannot suck up any water while you can, in a pinch, vac a bathed horse with a shop vac. It will help dry them in a hurry or you can plug the hose into the outlet for a cool air blower<G>.
Take it slowly when introducing your horses to the vac. Turn it on and don't use it on them when grooming...at whatever distance they can cope with. When you can finally run it right up near to them, introduce the hose, both with the vac on and off. Some horses are initially scared of the hose, even without the noise.
The first spot to use the vac on is your horse's favorite skritch spot, just rub it around on that spot...don't push the issue by moving it around too much or by taking it off and onto the horse's body...not until they are comfortable with it. Slowly is the best way to start. Some horses take to it right away, some take longer so let your horse be the guide in this so that they will learn to accept and love it.
If I can make a few suggestions to make the process easier, once they accept the vac (and I use a big 10hp one, so slize doesn't really matter other than noise). You can go to a big box pet store (or online to a big tack supplier) and get an attachment for the horses (same as the one for dogs but with a different sticky lable on it <G>). If the attachement isn't the same size as the end on your hose, you can get the right size end from the central vac people....also, you can get a longer hose from them (they sell it by the foot) which might make your job easier as you can then reach around the horse without dragging the machine too much.
I have taken this one step further, in so far as I have placed the vac is upstairs above the grooming stall, the long hose passes down through a hole in the ceiling in the center of the stall and never drags on the floor. I then have the vac plugged into a switched plug but you can also pass the cord down through the ceiling and just plug it in when you need it. The sound is less, but that doesn't seem to matter...I just find it easier....but the portable kind is nice as you can use it anywhere, even in their stalls.
Good Luck.