Convert a multiband raster to a polygon in qgis

I just figured it out... Use Raster Calculator:

band@1 * 0 + band@2 * 0 + band@3 * 0

Then will get a raster of two values, then vectorizing this raster will only take a second.

Try these steps, it might not be the best but it serves the purpose. It is based also on vectorization of Raster but with additional step:

1) Resample the Raster image to a lowest resolution. follow below link for example http://dominoc925.blogspot.sg/2015/04/resampling-single-geotiff-image-in-qgis.html
2) Use Raster to vector feature in QGIS.
https://docs.qgis.org/2.6/en/docs/training_manual/complete_analysis/raster_to_vector.html
3) Use the Dissolve feature to merge all polygons and form one final polygon.

You mentioned you have hundreds of files, why not using the Model builder to automate the process:
http://www.qgistutorials.com/en/docs/processing_graphical_modeler.html