Can I annotate my computer screen from the Vibe board?

Yes!

When you are screen casting via AirPlay, ChromeCast, or even HDMI, click the annotate/screenshot button (pencil icon) in the top right corner. This will take a screenshot of whatever is being displayed on the screen at the moment of clicking the button, and give you the ability to annotate the screen. You can then either save the annotations directly to the Vibe canvas opened on the board, or delete your edits and continue screen sharing.