As one of Vuo's developers, I work on Vuo's engine (the thing that makes compositions run), work on nodes, and write documentation. You'll see me on the forums answering people's questions about Vuo.
I've been developing apps and frameworks for several years (since I was in college). Pre-Vuo projects include Kineme Quartz Composer plugins, iOS apps for education, an app that analyzes photographs of tomato slices, and software to help people with disabilities use talking keyboards.
I enjoy using Vuo to make live music visuals. My hope for Vuo is that it will grow into a community of people of diverse backgrounds and identities making lots of different artistic, useful, unique, goofy, beautiful, crafty, wonderful compositions.
The format is only read at the start of the recording. (It's a constraint of the underlying macOS framework we're using for movie recording, AVFoundation.) Hm, we should mention format in the node's documentation, in the paragraph about "Certain characteristics of the movie are determined when saving starts, and can’t be changed while saving is in progress…"
Were you just experimenting, or was there something specific you wanted to use variable bitrate for?