I was at a similar cross roads - Didn't have anywhere near the A levels I needed and had the choice of doing another degree first or resitting. I chose to resit and had a few years out doing that and building up my application (work experience etc.) and would strongly recommend A level resits over another degree. 5 years at university is A LONG TIME - don't make it 8 if you dont need to!. Also consider the prospects of funding the second vet degree.
Having just graduated as a vet I would stress enough the length of time to study really wears you down being 5 years nevermind more.