Here is what I need to do:
Make a screencast software demonstration that will be uploaded to Vimeo in 16:9. I'm using Mac.
A couple of questions:
. How do I create a 16:9 composition? I'll need to add a Chromokey presenter over the screencast video, but I am okay with that bit. Just not sure how to get the basic 16:9 composition in the first place.
. What render settings should I then use to be able to upload it to Vimeo in 16:9, high quality?
It depends on whether you want to have a SD (NTSC/PAL) or HD 16:9 Composition?
It depends on the format/size/resolution of your source files.
For SD, you just pick a NTSC DV Widescreen or PAL DV Widescreen Composition preset, depending on if your source footage is NTSC or PAL.
For HD, most flavors are 16:9. If you shot HD, you need to find out if it's 720 or 1080 and which frame rate, then pick the matching Comp preset in the New Composition dialog.
The resolutions supported by Vimeo are listed here. It doesn't look like any of these correspond to presets in AE, but setting up the composition manually shouldn't be difficult. If you are posting HD video, create a composition that is 1280x720 with square pixels at 29.97 and you should be fine.
That page also tells you what compressor to use for video and audio. You might want to read up on the best ways to compress H.264 video, though, as there are several avenues for doing this and quality can vary accordingly.