Twixtor has arrived as a Vegas Pro 10 plugin. Unfortunately, it seems so watered down in features... I'm not feeling it's worth the $265 introductory cost.
I performed 2 tests.... slomoing a clip of guys diving into the ocean to 10% speed. Even when the divers had a very simple background (ocean water), and did not interfere with each other, twixtor created terrible artifacts around them throughout their flight through the air.... Even when I tweaked a few of the limited controls in the plugin, these artifacts remained. I don't see how I can use Twixtor to get results even close to professional. I see that the Twixtor plugin for AE has a Pro version in which you can set control points and separate your shot into layers, but none of this exists in the Vegas version.
The second test I did involved trying to create a morph between two faces... Unfortunately, without the ability to 'help' guide Twixtor, it chooses random points from each image to morph into the other... making it useless in trying to create a morph.
Maybe there are some special cases in which Twixtor will create a clean interpolation of frames, but I don't currently see how it's possible with the plugin for Vegas. I'll wager ALL of the demo videos showing cool Twixtor output are created with the 'Pro' version in AE with very specific source material.