I'm using a 2nd PC now, and there's a bit more info. Not sure why my other one isn't popping up this dialog, but anyway, this is what happens when I drop the track onto the timeline:
Compound Plug-In
Exception 0xCCCCFFFE (Unknown Type) IP:0x00007FF8EAC48B9C
In Module 'KERNELBASE.dll' at Address 0x00007FF8EAC40000 + 0x0000000000008B9C
Thread: compoundplug ID=0x12DC Stack=
I was able to work around the problem. I transcoded the source files first using Handbrake to H.264 mp4 files, and then dropped those in Vegas instead.
I can drop AVCHD files from my Panasonic AF-100 on the timeline with absolutely no problems, so it's not the codec itself. Very bizarre that a camera made by Sony won't work with Sony software.