I hear ya … no shortcut that I could find to replace a media event in the trimmer with the “next” one in sequence.
Two other ways .. Drag your media into the open trimmer window. It will replace the media that is currently in there. You can drag the media from the project window or the explorer window. Could be the fastest way.
Or .. (from Vegas help) You can automatically load a file into the Trimmer window by selecting the Double-click on media file loads into Trimmer instead of tracks check box on the General tab in the Preferences dialog. When this check box is selected, no events are created when you double-click a media file in the Vegas Pro Explorer, Media Manager, or Project Media window. Instead, the Trimmer is displayed to allow you to choose the portion of the file you want to use.
The last suggestion might be the best but I have not tried them.