We have versions of these logos as PNG (in various resolutions), SVG, and PDF. The Vega and Vega-Lite logos have monochrome black and white variants. You can find them all at https://github.com/vega/logos/tree/master/assets.
The Vega Project favicon is at https://github.com/vega/vega.github.io/blob/master/favicon.ico.
<link rel="shortcut icon" href="https://vega.github.io/favicon.ico" />