How Does QGIS Read 'nodata' in 3 Band Raster Data?

I took an orthophoto and warped it to have black slivers. In this state the new image is without nodata as can be verified with gdalinfo.

Band 1 Block=1000x2 Type=Byte, ColorInterp=Red
Band 2 Block=1000x2 Type=Byte, ColorInterp=Green
Band 3 Block=1000x2 Type=Byte, ColorInterp=Blue

Then I used your gdalbuildvrt command

gdalbuildvrt -srcnodata "0 0 0" out.vrt test.tif

The resulting .vrt file has now noData defined and gdalinfo reports it

Band 1 Block=128x128 Type=Byte, ColorInterp=Red
  NoData Value=0
Band 2 Block=128x128 Type=Byte, ColorInterp=Green
  NoData Value=0
Band 3 Block=128x128 Type=Byte, ColorInterp=Blue
  NoData Value=0

This is how the warped image shows in QGIS with default settings on top of a green polygon

And this is how the VRT file with nodata opens in QGIS out-of-the-box

I can't see any issue with my test image. If you still have a problem it would be nice if you could provide us with test data.

Here are two images representing what got us to questioning how nodata is read in QGIS.

When we defining the nodata of the image to "0" using QGIS, we were also making internal pixels transparent too. Even those regions that were not "0 0 0" are being rendered transparent.

I was only using QGIS as my testing space to be honest, but it was helping me understand how to solve the issue using MapnikXML; which we managed to do using a combination of:

gdalbuildvrt -srcnodata "0 0 0" welly.vrt *.tif

then rendering the individual rgb bands in Mapnik and adding them back together with:

raster-comp-op: plus;

Using CartoCSS, in order to achieve a similar outcome to the solution in QGIS seen above, the code looks something like:

#red 
{
  raster-opacity:1;
  raster-scaling:bilinear;
  raster-comp-op:plus;
  raster-colorizer-default-mode:linear;
  raster-colorizer-default-color: transparent;
  raster-colorizer-stops:
    stop(0,transparent, exact)
    stop(1,#000)
    stop(200,rgb(255,0,0))
}


#green 
{
  raster-opacity:1;
  raster-scaling:bilinear;
  raster-comp-op:plus;
  raster-colorizer-default-mode:linear;
  raster-colorizer-default-color: transparent;
  raster-colorizer-stops:
    stop(0,transparent, exact)
    stop(1,#000)
    stop(200,rgb(0,255,0))
}

#blue 
{
  raster-opacity:1;
  raster-scaling:bilinear;
  raster-comp-op:plus;
  raster-colorizer-default-mode:linear;
  raster-colorizer-default-color: transparent;
  raster-colorizer-stops:
    stop(0,transparent, exact)
    stop(1,#000)
    stop(200,rgb(0,0,255));
}