You aren't requesting anything from the url, you're trying to json_decode
the url itself.
Instead try something like:
$json_url = 'http://api.steampowered.com/ISteamUser/ResolveVanityURL/v0001/?key=' . $devKey . '&vanityurl=' . $username;
$res = file_get_contents($json_url);
$json_output = json_decode ( $res, true );
var_dump($json_output);
Also have a look into curl
as this is more powerful when dealing with apis.