Someone promoting himself as being more deserving of help for any reason, including being a vet, would make me less sympathetic. My reasons are my own and if I want to give someone money then I will. I don't like someone telling me I should give them money because of X, with the implication that if I don't I'm somehow a bad person.
Same goes for the give to charity guilt trip. Just because I don't give money for cancer research or children born without elbows doesn't mean I'm a big fan of cancer or using stiff armed kids for target practice.
And if the guy has all 4 limbs still attached, forget it. Doesn't matter how many flags he wraps himself up in.