VSE render black as transparent July 5th lots of fireworks vids. Using VSE I would like to overlay some of them on each other.  So convert all the black in the scene to transparent then render it out and bring the .png files back in and over lap the fireworks on top of each other...
Using nodes in the Compositor. 
I have seen some tutorials added the Movie and Composite output setup a colorramp off the clip to the AlphaOver node Loc; with Image1 full transparent and in image two the movie clip image.
The viewer shows the fireworks on transparent background.
output .png ; for 24 frames; click animate and all the png rendered with black where the viewer shows transparent.. no alpha channel in rendered output.
I'm a bit confused here, did you want to composite the images using the VSE or the compositor? No need to do it twice, either section is capable of doing what you're asking.

If the background is completely black you don't need to change the alpha.
A common way of compositing images with dark backgrounds is to mix them via luma keying.
You can try to add alpha channels, but an easier way for this application would be to just add the clips.

In the VSE this can be done by selecting the clips above your base clip and setting the blend mode to add instead of cross.



In the node editor add a color->mix node and set the mode to add.
Hope that helps.