A week ago, I shot some footage under really bad weather conditions... I thought this was a good one to give a try to replace the sky by some blue sky with some clouds (using generated media). To achieve this, I used luma keying in Vegas. If someone is interested in the results that can be gotten this way, I have posted an example here. If anyone is wondering how luma keying is done in Vegas: use the secondary color corrector, enable only luma, apply no colour change and set alpha to 0.