Go to Effects Tab > Matte > Widescreen and apply the Widescreen filter to the clip(s) on your timeline. By opening the clip on the timeline you've applied the effect to in the viewer, by going to the Filters Tab, you can adjust the appearance of the widescreen:
This will result in a letterboxed widescreen, which some people prefer as it really highlights the widescreen by surrounding it with black borders. If you want the finished video to appear without letterboxing, when you complete your edit, export a QuickTime movie of your completed sequence (without conversion) and take that file into Compressor. When you use Compressor to convert it to H.264 for Vimeo, you can also re-crop it so that resultant file is natively widescreen, not letterboxed.