I think you can get up to 1200 times speed by ctrl-dragging the edge of the clip. Drag it to it's smallest size, and that's around 1200x speed. The envelope still only shows 400, but it's relative to the size you have dragged it to. Somebody correct me if I'm wrong.
There are two ways that you can do this, one with Vegas, and another way using a program called scenealyzer. In scenealyzer you can set it to capture 1 out of every N frames, during the capture process. so set it to one out of every 100 or so frames. This will give you extremely fast motion. If you must only use vegas, set it to the 12x speed as described above, render out that clip, re-import it and again set the playback speed to 12x, repeat as necesary.
You'll get a cleaner image using the sped up/rendered file than you will with Scenelyzer, but if you're taking it faster than 24 times, then the audience likely won't be able to see the diff.