I did a 90fps in game acquisition and then set at 30fps the source file setting in vdub settings.
-> the avi generated is 3 times slower than the original action.
-> use the velocity enveloppe in vegas to accelerate the clip 3 times and get the orginal lenght of your action. It will result in a motion blur effect.

NB : you will not be able to put more any fast forward in your movie since Vegas is limited to 300%. But you'll be able to make really smooth slowmotion.

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You can also set virtualdub to 90fps as file source and let 30fps in Vegas properties. It will result in the same motion blur with fast forward possibilities.