I cloned the official OpenSeeFace source and ran its native Python tracker under Linux. Using Python 3.10 with the required dependencies, this command works:
python facetracker.py -c 0 -W 1280 -H 720 -F 30
It successfully accesses /dev/video0, detects and tracks the face, and sends tracking data to the default OpenSeeFace destination: 127.0.0.1:11573
With this running, vTuber Kit immediately responds to my face. The webcam and Unity OpenSee UDP receiver both work correctly on Linux. The problem appears isolated to the OpenSeeLauncher/bundled OpenSeeFace tracker portion of the Linux build.
The existing Unity OpenSee receiver already works on Linux, so the simplest fix may be to provide a Linux-compatible OpenSeeFace tracker and have the Linux build launch that instead of the Windows facetracker.exe. No changes to the vTuber Kit UDP receiver were required for this workaround.