Try this script by James Trietsch, he's an awsome programmer!:
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/**********
* Zero Smooth - Sets all video keyframe smoothness properties to zero
* by James Trietsch
* (A modification of match_aspect_ratio.js)
**********/

import Sony.Vegas;
import System.Windows.Forms;

var tCount = Vegas.Project.Tracks.Count;

for (var i=0; i < tCount; i++) {
var track = Vegas.Project.Tracks[i];

if (!track.IsVideo())
continue;

var events = new Enumerator(track.Events);

while (!events.atEnd()) {
var tEvent = events.item();
var mStream = GetActiveMediaStream(tEvent);

if (mStream) {
var vStream = VideoStream(mStream);
var keys = VideoEvent(tEvent).VideoMotion.Keyframes;
var kCount = keys.Count;

for (var j=0; j<kCount; j++) {
keys[j].Smoothness = 0;
}
}

events.moveNext();
}
}


// Copied from Match Aspect Ratio script
function GetActiveMediaStream (trackEvent : TrackEvent)
{
try
{
if ( ! trackEvent.ActiveTake.IsValid())
{
throw "empty or invalid take";
}

var media = Vegas.Project.MediaPool.Find (trackEvent.ActiveTake.MediaPath);

if (null == media)
{
throw "missing media";
}

var mediaStream = media.Streams.GetItemByMediaType (MediaType.Video, trackEvent.ActiveTake.StreamIndex);

return mediaStream;
}
catch (e)
{
//MessageBox.Show(e);
return null;
}
}

There is a similar script here:

UnSmooth Keyframes

Thanks for everyones help but it still doesnt work. It really seems to be linked to the scaling (and rotation for that matter) rather than the translation of the crop.

. When I crop and animate a pan I can actually contol the smothness of the keys. I can have the pan start abruptly (linear) and stop gradually (smooth).
. When I want to animate the size of the crop (scaling), I cant seem to be able to have the pan start linear and end smoothly (gradually). Its either all smoothed or or linear.

The two examples above have just 2 keys for example sake.
After extensive tests (fiddling around with the smoothness parameter, trying out all of the interpolation modes), it seems that its the animated resizing and rotation that doesnt work as it should.
I'm starting to wonder if that isnt linked to the fact that only POSITION track key is available in Event Pan/Crop (And by the way, it would be cool to have different tracks for rotation, scaling and whatever.And also, I'm dreaming, a proper graph editor).

If anyone else has some ideas...
Thansk a lot,

Beugnet