I have this problem. It's weird though...
I installed Windows Live Messenger as the first thing after Vista x64, and the plugin was there, worked fine. Worked fine for a few days, then it just suddenly disappeared yesterday morning. It was back this morning while I was at school, but when I got home and opened up my laptop, it was gone again.
I don't know why it does this. Any ideas?
Vista Ultimate x64, 1.6Ghz AMD Turion64 x2, 1gb RAM, 120gb hard drive.
Don't know if that helps, but that's my laptops spec.