For the responsive theme, I'm using the method shown here to scale the player.  It uses percentages to determine how tall the video should be.  From testing on a widescreen video from Vimeo I found that 58% was about right.  However, if you have a video that isn't in this ratio you will need to update the css.

1. Copy the responsive template from /packages/vimeo/blocks/vimeo/templates/responsive to /blocks/vimeo/templates/your_template_name
2. Choose your new template from the custom template dialog
3. Edit the view.css - change the value for padding-bottom on the .vimeo-embed-container class to be the value you need.

That should be all you need to do to get responsive videos in the proportions you need.