Perhaps you should try some spelling variations:
"dong sac dan van." » "dong sach dan van." (Expand Search), "dong bac dan van." (Expand Search), "dong cac dan van." (Expand Search), "dong nhac dan van." (Expand Search), "dong sac san van." (Expand Search), "dong sac tan van." (Expand Search), "dong sac ban van." (Expand Search)
"cong tac doan van." » "cong tap doan van." (Expand Search), "cong tac dan van." (Expand Search), "cong tai doan van." (Expand Search), "cong tac doan phan." (Expand Search), "cong tac luan van." (Expand Search), "cong tac san van." (Expand Search)
"cong tac dang van." » "cong tac dong van." (Expand Search), "cong tac dan van." (Expand Search), "cong tac dung van." (Expand Search), "cong tac hoang van." (Expand Search)

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• Performing a fuzzy search may retrieve terms with similar spellings: (("dong sac dan van.") OR ((("cong tac doan van.") OR ("cong tac dang van."))))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"dong sac dan van.\") OR (((\"cong tac doan van.\") OR (\"cong tac dang van.\"))))".
• Removing quotes may allow a broader search: (( dong sac dan van. ) OR ((( cong tac doan van. ) OR ( cong tac dang van. )))).
• Adding a wildcard symbol may retrieve word variants: (("dong sac dan van.") OR ((("cong tac doan van.") OR ("cong tac dang van."))))*.