No idea as from where the surname might originate but happy to give few websites to help in your search.
I am sure you know that you start the family tree starting with yourself and going backwards untill you come upon an ancestor that is from the "old country". The church records of town of Siret should be of some help. You might want to hire a researcher in Romania to do the research if the LDS-Mormons (Family History Center) website is not listing any church records.
Romania's phone site is listing the surname MANDIUC. www.whitepages.ro www.rat.de/kuijsten/navigator http://vivisimo.comhttp://vivisimo.com www.imrehun.com www.centroconsult.sk