Perhaps you should try some spelling variations:
vu khach » vu khanh, vu khac, vay khach
hieu van » hieu va, thien van, lieu va
cac nuoc » cap nuoc, hoc nuoc, dat nuoc
hanh dai » hanh tai, hanh dao, thanh dat
vanh dai » van dai, vanh tai, lanh dao
ky lang » ky dang, ky tang, tay lang
van cac » van hac, van cau, van bac
cua tre » cua te, cuu tre, cua triet

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

• Performing a fuzzy search may retrieve terms with similar spellings: vu khach ((ky lang) or (ky nang)) xa hoi. hieu van cac nuoc va ((hanh dai) OR (vanh dai)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: vu khach ((ky lang) OR (ky nang)) xa hoi. hieu van cac nuoc va ((hanh dai) OR (vanh dai)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "vu khach ((ky lang) or (ky nang)) xa hoi. hieu van cac nuoc va ((hanh dai) OR (vanh dai)) cua tre".
• Adding a wildcard symbol may retrieve word variants: vu khach ((ky lang) or (ky nang)) xa hoi. hieu van cac nuoc va ((hanh dai) OR (vanh dai)) cua tre*.