With D grades your application will get an immediate rejection. And, unless there are compelling reasons why you did that badly, then in all honestly Vet probably isn't the career for you. The course is 5 years long and academically demanding - especially in hard science like Chemistry. If you are struggling at A level then this probably is not the course or career for you. Most Vet Schools have a strict policy about resits - they don't need to be kind to people who can't get top grades at first attempt as there are enough people out there who can.
Graduate entry is not a realistic option as you will still need stonker Science A levels - there are not many places and competition is fierce. And as its a 2nd undergrad degree, you'd have to fund the entire 5 years of the course yourself (both high fees and maintenance).
Realistically, you need to set your sights lower than Vet Med. Look at Vet Nursing, Biovet Science, Zoology, Wildlife Conservation etc. You will probably still need to resit Chemistry and possibly also Biology, depending on the course requirements.