user14465: I am a beginner to Adobe After Effects and Sony Vegas Pro, so forgive me for a lack of a better term.
I am trying to create an Adobe After Effects composition. I want the composition to be transparent with a blur effect. I want to add this blur effect to be in such a way that upon exporting it to an .avi file, I can add this .avi file to a Sony Vegas Project, and the .avi file will blur any layers underneath it.
To illustrate, suppose I have the following Sony Vegas Project:

I want to create an after effects composition such that I can make the image look something like this:

However, I don't want to render the composition with the image in it. I want to render it as an .avi file such that I can apply the blur effect to any arbitrary image, or even video files.

Is this possible? My intuition tells me no, but I'm asking in case there is a way.

user3481: This is not possible, because the blur effect needs the underlying video as input.
You can either create the blur effect in Sony Vegas without the need for After Effects, or you can (for more complex effects) import the video into After Effects, add your effects, render the result as video, and import it into Vegas.
As you can see the second option takes some time and is only worth it for complex effects/animations that you cannot do in Vegas. It's better to try the included effects of Vegas first. 

user5347: You won't be able to create a clip that can 'blur any layer underneath' when placed in other applications.
You can just blur assets from within AE and export them out.

user1871: It's not possible. A generic blur effect relies on a function which smooths out pixel values within a specified radius. A mask combines the value of a pixel in the top layer with the pixel of the layer beneath, modulated by the corresponding alpha value of that pixel in the top layer.
So, in short a blur is a 'lateral' function, whereas layer compositing is a 'depth' function.

14465: <p>I am a beginner to Adobe After Effects and Sony Vegas Pro, so forgive me for a lack of a better term.</p>

<p>I am trying to create an Adobe After Effects composition. I want the composition to be transparent with a blur effect. I want to add this blur effect to be in such a way that upon exporting it to an .avi file, I can add this .avi file to a Sony Vegas Project, and the .avi file will blur any layers underneath it.</p>

<p>To illustrate, suppose I have the following Sony Vegas Project:</p>

<p><a href="https://i.stack.imgur.com/FMw9d.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/FMw9d.jpg" alt="enter image description here"></a></p>

<p>I want to create an after effects composition such that I can make the image look something like this:</p>

<p><a href="https://i.stack.imgur.com/kB98j.png" rel="noreferrer"><img src="https://i.stack.imgur.com/kB98j.png" alt="enter image description here"></a></p>

<p>However, I don't want to render the composition with the image in it. I want to render it as an .avi file such that I can apply the blur effect to <em>any arbitrary image</em>, or even video files.</p>

<p><a href="https://i.stack.imgur.com/aovkH.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/aovkH.jpg" alt="enter image description here"></a></p>

<p>Is this possible? My intuition tells me no, but I'm asking in case there is a way.</p>
 3481: <p>This is not possible, because the blur effect needs the underlying video as input.</p>

<p>You can either create the blur effect in Sony Vegas without the need for After Effects, or you can (for more complex effects) import the video into After Effects, add your effects, render the result as video, and import it into Vegas.</p>

<p>As you can see the second option takes some time and is only worth it for complex effects/animations that you cannot do in Vegas. It's better to try the included effects of Vegas first. </p>
 5347: <p>You won't be able to create a clip that can 'blur any layer underneath' when placed in other applications.</p>

<p>You can just blur assets from within AE and export them out.</p>
 1871: <p>It's not possible. A generic blur effect relies on a function which smooths out pixel values within a specified radius. A mask combines the value of a pixel in the top layer with the pixel of the layer beneath, modulated by the corresponding alpha value of that pixel in the top layer.</p>

<p>So, in short a blur is a 'lateral' function, whereas layer compositing is a 'depth' function.</p>
