Good question! I have also been curious if it's okay to give your cat vitamins. I haven't had time to schedule an appointment with my vet, but I'll be sure to ask her when I do!
http://www.catnipbazaar.com/cat-vitamin-supplement
Cystease is a product that contains 125mg of glucosamine. Renalzin is sold as a phosphate binder and according to the label to be used for feline kidney failure patients. Renalzin contains Lanthanium carbonate as the primary ingredient along with Kaolin and Vitamin E.
These two products are not similar at all and should not be substituted. If your cat is not in kidney failure then Renalzin would not be indicated. Aluminum hydroxide, rather than Renalzin is the phosphate binder of choice to eliminate excess phosphorus in cats with chronic kidney failure.
For cats with FLUTD, we have found that a couple natural cat vitamin supplements with glucosamine, MSM, cranberry extract, etc have proven quite helpful to eliminate recurrences of lower urinary tract infections in cats like yours. Should you want to follow up on this, it would be my pleasure.
With respect to your cats food, home made diets are excellent, provide real, honest nutrients and are quite easy to prepare and very cost effective.
Dr Carol Osborne, DVM