I don't think Vader gave an order to kill the Lars'... Like Thebumwhowalks, anyone who came into contact with the droids was to be "taken care of", and they wouldn't have needed Vader permission to kill the Lars Family, because they already had it. When he was Anakin Skywalker, he never felt any anger toward the Lars, they freed his mother for goodness sake. I feel like if he had known, they would have been spared. 
It's also a lawless planet on the fringes of the galaxy that the Empire had very little control over, the Stormtroopers could have gotten away with ANYTHING.
edit- well, looked it up on wookiepedia and apparently he did order their death, but it's from some EU book, which means you can accept it as part of canon or not, if you so choose. To me, Vader isn't pure evil, and that right there is pure evil. To me, Vader seems like a very confused man, with great power, being influenced by someone who is pure evil.