The implications of this line of reasoning are not apparent at first blush. Does the person making the argument wish to imply that vegetarianism leads to National Socialism, or that National Socialism leads to vegetarianism? Perhaps we are to infer that because Hitler supposedly loved animals, we have a moral obligation to hate them? Whatever the implication might be, it's clear that the assertion that Hitler was a vegetarian is a brazen attempt to Poison The Well of abstinence from animal foods by associating the practice with the most reviled, but by no means the most murderous (that distinction goes to the meat-eating Joseph Stalin), man of the twentieth century.

Even if it could be proved that Hitler had been a vegetarian, the fact would say nothing at all about the underlying tenets of veganism. And in any case, the claim appears to be quite untrue. According to Robert Payne, who wrote what is considered by many, the definitive biography of Adolf Hitler, the Fuhrer had a well-attested fondness for Bavarian sausage, caviar, cream, eggs, and stuffed squab. In no sense of the word “vegetarian” (to say nothing of “vegan”) are sausage, caviar, and squab permissible.

The notion that Hitler was a vegetarian was a gross fabrication conceived and promulgated by his minister of propaganda, Joseph Goebbels, who wanted Hitler to be seen as an ascetic, a paragon of self-restraint, a man invulnerable to the myriad temptations of the flesh, who did not drink alcohol, smoke cigarettes, consort with women, or eat meat. While there is apparently no evidence to suggest that Hitler ever smoked, there is also no question that he drank alcohol, kept the company of women, and regularly ate meat and other animal by-products.

From Payne's book, The Life and Death of Adolf Hitler:

Hitler's asceticism played an important part in the image he projected over Germany. According to the widely believed legend, he neither smoked nor drank, nor did he eat meat or have anything to do with women. Only the first was true. He drank beer and diluted wine frequently, had a special fondness for Bavarian sausages and kept a mistress, Eva Braun, who lived with him quietly in the Berghof. There had been other discreet affairs with women. His asceticism was fiction invented by Goebbels to emphasize his total dedication, his self-control, the distance that separated him from other men. By this outward show of asceticism, he could claim that he was dedicated to the service of his people.

The fact that over six decades after his death, many people still believe the fictitious details of Hitler's public persona testifies to the effectiveness of Goebbels' propagandizing. But the mere persistence of a lie does not bestow any measure of truth upon it. Even less does it undermine the ethical or moral justification of the philosophy around which the lie is centered.

Your argument is invalid.

Sources:

http://www.vegsource.com/berry/hitler.html

http://www.vegsource.com/berry/hitler_book.htm

Robert Payne, The Life and Death of Adolf Hitler (New York: Praeger, 1973), pp. 346-7.