Userscripts.org

This is the only script for playing Youtube videos in VLC that will work in SeaMonkey with Scriptish installed.
Any others are otherwise broken or freeze/crash VLC. This one works every time.
The only complaints:
1. It doesn't let you seek to a specific part of the video (only with x-flv)
2. It doesn't give you a buffering indicator telling you how much of the video is playable. (it buffers pretty damn quick with mp4 and webm - flv is probably an old format.)
Otherwise it works like a charm and is excellent.