Checking an 1867 dictionary (from our collection of nineteenth-century dictionaries--this one just happened to be at hand):
"Bivouac" is as French word meaning "the guard or watch of a whole army, or an encampment without [emphasis added] tents. It has an eighteenth-century origin. It may have been pronounced "bi-vwa" with a short "i" sound. The crude translation is "be at wake" or be at watch." I believe the "ac" is a mis-prounciation that has been carried forward.
None of our dictionaries have the "ack" spelling.
When in doubt ask a librarian--I am married to one.
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