Are you speaking for yourself there? Because I reckon most people have long since made up their mind about Cameron, and it's only a very narrow section of vociferously pro-EU Tory supporters that are likely to change their opinion of him if he fails to win this referendum.
Not that I think he's deliberately throwing this referendum or anything. He might not lose much from losing, but he sure as hell wouldn't gain anything from it.
I don't think they will redefine it, but us leaving the EU will. Or, rather, look at it this way.
At the start of the season Spurs lost our opening game to Man Utd thanks to an own goal. "What a Spursy start to the season" we say. The next three or four games go without a win, though we do draw them all - against teams we should have been beating. "This is dire" we say. "Why can't we break down these teams?!" We're languishing three quarters of the way down the league. Fast forward to our game against Newcastle at the end of October and by this point we're doing much better! We haven't lost a game since the opening day and
even that was a soft loss thanks to an own goal!" Fast forward to now - we're 2nd in the league, we have the strongest defensive record in the league and we're competing for the title. Now people look back on our start and show it off as a record of strong defending and not giving in to pressure.
The games haven't changed, the results are the same as they ever were. But, due to what came afterwards, the way they're framed in the narrative and looked back on is totally different. What started as our rough opening patch where we couldn't win becomes a defiant sign of a strong defense when the record later shifts. I think this is the risk Cameron faces - you can view most things two ways, and if he manages to keep us in the EU - effectively
stopping his historically-out-of-touch-on-the-EU party from ripping up the country and their own government - then his ploy to keep UKIP at bay during the election will be seen as cunning political play that helped lead to a majority. If not, it'll be seen as a reckless gamble by a PM who would risk anything not to be seen as a one-term PM presiding over a coalition.
I'll not give an in-depth reply because it's off-topic, but if you think that's a 'legacy' he'll struggle to protect then you have a much lower opinion of him than I do.
(I, at least, give him credit for being an intelligent sneak, rather than an honest fool.)
I think we all like to imagine that we have more control over our legacy than we really do. It won't be up to Cameron what people think of him when he's gone.