I use Magix Movie Studio 13 Platinum, it has the power of Vegas but at a fraction the cost. If you decide to get this, make sure you get the Platinum version. The normal version doesn't allow advanced editing.
Free and lightweight is a tall order. Virtualdub and Windows Movie Maker are the only two I know of that isn't limited by some artificial restriction. Last time I tried virtualdub it was just so buggy and crashed constantly. I hope it has improved over the years.
There is also Lightworks. This is professional editing software with a learning cliff to match. It extremely expensive. However there is a free version but it is limited to exporting 720p.