Since today there exists a pure Rust GStreamer decoder plugin for Vorbis, using the lewton Vorbis decoder crate.
This should be more or less feature-equivalent with the C, libvorbis-based, decoder plugin from gst-plugins-base.
https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/tree/master/gst-plugin-lewton
If someone looks for a smallish project to get started with GStreamer in Rust, a FLAC decoder around claxon or an Y4M encoder/decoder around y4m seem like good starter tasks.
See
https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/issues/71
https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/issues/61
Feel free to ask for more details / help!