Good all-around vitamin & probiotic combo - does it exist?

With all the info on the internet, I’m getting lost in all the hype, reviews, etc. about vitamins and probiotics. Without going into the novel that is my 3 yr old dog and after finally figuring out protein allergies/tummy issues, we’ve had almost a year since I adopted her that we haven’t had any digestive problems. Due to her very restricted vet diet that I can only add veggies to for variety, I want to make sure she is getting as well-rounded a diet considering its limitations. Her prescription dog food is supposed to meet nutritional standards and has a limited amount of probiotics. Is there a single supplement that would be beneficial to add that would have a bio-available multi vit/min AND probiotics at a higher level? Or is this just throwing money to the wind as in: It might not help, but can’t hurt category? Again, not currently dealing with any digestive issues, just want to make sure I’m doing the best I can by my girl. I had planned to do a real food diet, but that is now out of the cards. Thanks!!