Definitely file a complaint with the DOT here:
http://airconsumer.dot.gov/escomplaint/es.cfm
This was obviously an involuntarily denied boarding and you should have been advised by Delta of your rights as far as compensation. You should have been issued a check on the spot. Unfortunately when you accepted the vouchers Delta probably had you sign a statement releasing them of any further obligation. You should claim that given the circumstances you were confused and that the vouchers were signed for under duress.