Dehydrating Apple Cider Vinegar?
You might actually want to look at other methods - either alongside or instead of your current dehydration. 

"Commercial" Vinegar powder (where it isn't actually just the acid components) is basically maltodextrin  sprayed with your vinegar and dried. There's a ton of references like this one, though admittedly I learnt this from watching how its made.  

The nice thing here is you don't really need more heat (or high heat) to do this.  In theory you might also be able to do it with starch.

Since the water is 'locked away' its also less likely to dissolve your capsule 

I'm almost half certain that I remember that spray drying or freeze drying would work too, but I can't find any sources, and these arn't processes one can easily do at home