I had the same problem, and took the advice from the other people on this post….until!!!! I found out that in fact it wasn’t my computer at all.. Its all about the codecs buddy.
Try this…Splash Player lite
You will find that it plays your video’s fine, but still doesn’t sort your Sony Vegas problems out.
I still have the same problem, but at least it stops you from taking advice and spending on a new graphics card and processor when in fact its more of a software issue
Hope this bends light in a new direction 🙂