Does anyone know the API or SDK for developing your own Sony Vegas plugins?

Not sure if this is what you want but try Sony Development Kits (http://www.sonycreativesoftware.com/download/devkits).

Thanks, Mike. Unfortunately, most of those are for writing Vegas scripts. There is one Video Plug-In Kit, but that is some kind of DirectX kit, which seems to have nothing to do with Vegas (it has no documentation either, just some sample software with no explanation, very strange).

I took a second look, and I think that is actually it. Thanks, Mike!

Adam, I'm glad to hear that it worked.
As I said, I had no idea whether it was what you wanted or not but it sounded good to me :-)

It worked so well that I have released my first plug-in (http://www.pantarheon.org/Bororo3D/). It is for editing 3D videos in Vegas, but it also includes a couple of transitions that are useful for both 3D and 2D. :)

So, thanks again, Mike.