"I think the vegans had far more to lose if they just silently stopped eating mars bars."Fair point. It is difficult enough to predict the effect of our actions, and it is often better to campaign in good faith for what you believe in, rather than worry about the (sometimes devious) strategies of other people. The well-publicized campaign (to restore vegetarian status to the Mars Bar) may have wider effects - for example on other food manufacturers that may wish to join the ranks of the Accidentally Vegan - products that are not specifically targeted at vegetarians or vegans, but just happen not to contain animal ingredients.
(Given that many of these accidentally vegan products are produced in factories alongside non-vegan products, the Accidentally Vegan folk are willing to tolerate trace amounts, which they justify in terms of the likely effect on animal welfare. Obviously if this bothers you, then you should stick to Deliberately and Certifiably Vegan. Or grow and cook your own food.)
There are undoubtedly many food products that are Accidentally Non-Vegan. In other words, they contain animal ingredients for no particularly good reason. Like a chef sprinkling bits of crispy bacon into a salad. (And then picking them out again when a vegetarian complains.) It is possible that the vegetarians could have a greater impact from a campaign in this area. But psychologists have long known that people will fight more strongly to retain or restore something they already have (or have had) than to gain something they have never had.
Meanwhile, back to the Mars Bar. I haven't eaten one myself since I was a child, but I understand from Wikipedia that the Mars Bar contains milk chocolate. So although it may be suitable for vegetarians, it surely cannot be suitable for vegans. Sorry Adam.