April 5 , 2019
one more note : that last project is not 100% , as it relies on using GPU 32 bit project settings .
( the previous update , march 24 , 2019 will NOT work using 8 bit project setting )
This update shows , with just one image , how to Chroma key cut a green\blue screen . That
act of using Chroma key , places the Mask directly into the alpha channel . Noting else to do .
Then export\render out to 32 bit .png or 32 bit Video for Windows . The exported media
contains the RGB of the cut ( background screen color will NOT be part of the export ) ,
with mask in the alpha channel .
Then test with Standard , Pre-Multiply , Pre-Multiply Dirty , to retrieve the best cut . Done .
( That was a lot of messing around , in the previous attempts )

March 24 , 2019
i just did it better , this is the general outline of what to do .
i will update the rest with new images and results ( i have successfully done it )

Track 1 contains the foreground RGB image . its' Alpha channel needs to be set to Solid Black .
Track 1 , uses FX Channel Blend , each Alpha is set to -10.00 ( track 1's alpha channel is now Solid Black )
Track 1 will uses ADD , blend mode .
Track 2 Parent of track 3 . track 2 holds Generated Media of Solid Black .
Track 2's alpha channel is Solid White .
Track 2 uses Track blend Mode Multiply , to track 3
Track 3 child of track 2 , Track 3 holds the image you want to insert into the Alpha channel of track 1 .
Track 3 , uses FX Channel Blend , set each Alpha digit box to 1.000

the above description effectively does this :
Track 1 holds the source RGB image , and it has an alpha channel of solid black
Track 2 multiplies with track 3 causing an RGB image result of solid Black , while it's alpha channel was previously stuffed
with the the RGB channels of track 2 .
Finally , the ADD of track 1 does this :
Track 1's RGB image is ADD to the RGB Solid Black image of track ( 2 3 composite ) . and also ADD's
the Black alpha channel of track 1 to the alpha channel of track ( 2 3 composite ) .
the result image has track 1 in the RGB channels , while track 3's RGB is in the alpha channel of the result image .

a quick summary of the above :
source image has been give a solid black alpha channel , while source image RGB is still full intact .
image 2 has been given solid black RGB channels , while those RGB channels have been placed in image 2's alpha channel .
when we ADD , black is 0 , thus track 1's RGB is added to 0s of image 2's RGB and we end up with track 1's RGB .
when we ADD , tracks 1's alpha channel is solid black( all 0's ) , is added to the alpha channel of track ( 2 3 composite ) , the
result of that is the alpha channel is now that of track 2's image

we need a :
source image that contains RGB image with Solid Black alpha channel
second image with RGB of Solid Black , with an image in the alpha channel

of those last 2 sentences , note how\where the "Solid Black" is swapped .
that might sound trivial , but to set of the working project was always illusive .
i was just re-looking at it , and will post a new project below with better pictures .

to get a 32 bit .png out of Vegas , use the Viewer . click top right icon to save image to file and choose .png .
also , set the quality of the viewer to Best-->Full before saving this image to file .
note , save to clip board icon , exports a 24 bit image to the clip board , so only use Save Viewer to File .


End Update March 24 , 2019

August 20 , 2017
you can now originate your own 32 bit video and images , with Alpha Channel , out of Vegas .

***
this isn't perfect . some situations may not produce the desired perfect results .
ie ; large black portions seem to be an issue .
the simely faces used very little black , thus this issue went un-noticed .
***

The main RGB image is the Front Gate of town "White Run" , from Skyrim .
a "Smiley Face" has been moved into the Alpha Channel .

The smiley face can now be used as a Mask .

After the 32 bit .png or 32 bit .avi video , has been rendered out ,...
when importing the 32 bit image , or 32 bit video ,
be sure to use "Pre-Multiply" , in Event-->Properties .
Right click the Event on the Time Line , and click Propertiies ,
then Media Tab , then set Alpha Channel to Pre-Multiply .