I just had a similar problem this week — twice. The first was after I did a large round of house cleaning — removing programs I wasn’t using, deleting files I didn’t need, etc. Then Vegas 8 stopped loading. It quit at the same place as yours, though the error message I was getting is different. I was getting a “The memory could not be read” message — weird thing is, it was being attributed to rundll32.exe, not Vegas itself.
I went through several hoops, but the one that got me back on track was uninstalling Vegas and removing all references to Vegas 8 from Windows’ registry. (Vegas 6 was working fine, so I didn’t touch any generic Vegas or Vegas 6 specific entries.) Then I reinstalled Vegas 8. I had tried uninstalling / reinstalling multiple times before that, but nothing helped until I cleaned out the registry.
It worked okay for a couple days, and then — after installing Windows Media Encoder 9 — I started getting the exact same error message when trying to run Vegas 8. I uninstalled WME, tried running Vegas, and got the message again — but this time, Vegas continued to load anyhow. And now the message doesn’t show up anymore. Why WME would cause this, I don’t know. I’m sure I’ve had it installed before without problem.
Not sure if my machine was having the same as yours, but hopefully this will provide some leads.
Rob