Perhaps you should try some spelling variations:
van doan » van doanh, van toan, van hoan
tang mai » tang ma, tang tai, lang mai
cam thuc » am thuc, nam thuc, cam thu
hieu ve » hieu va, hieu van
mai hoi » mai khoi, dai hoi, lai hoi
va doan » va doanh, va dan, va don
ve cam » ve cham, ve cac, va cam
sai ky » sac ky
vi hoa » voi hoa, vi hoc, va hoa

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: so ((cac ky) or (sai ky)) tang mai hoi. hieu ve cam thuc ((va doan) OR (van doan)) vi hoa viet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: so ((cac ky) OR (sai ky)) tang mai hoi. hieu ve cam thuc ((va doan) OR (van doan)) vi hoa viet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "so ((cac ky) or (sai ky)) tang mai hoi. hieu ve cam thuc ((va doan) OR (van doan)) vi hoa viet".
• Adding a wildcard symbol may retrieve word variants: so ((cac ky) or (sai ky)) tang mai hoi. hieu ve cam thuc ((va doan) OR (van doan)) vi hoa viet*.