First of all congratulations on having your kitty live to be 20 years old. You obviously are taking great care of her. It sounds like you are doing about as much as you can do.
The vomiting may be due to build up of uremic toxins. Without getting them flushed out, she may continue this.
Natural support for kidneys are astragalus, hawthorn, marshmallow, gotu kola, echinacea, burdock root, garlic, ginko biloba, B vitamins and kidney glandulars. My guess is much of this is in the renal support powder you are already giving her.
I would just make her as comfortable as possible and keep on loving her. If it turns into more of a crisis, you may have to get her to the vet.