Perhaps you should try some spelling variations:
vien cac » vien bach, vien dai, gian cac
tu sach » thu sach, su sach, te sach
sach ky » sac ky, sach y, sach ty
vien da » vien dai, hien dai
cua tre » cua te, cuu tre, cua triet
va dan » ca dan, va dao, va dau
dan vi » doan vi, don vi, dan va
vi cua » vai cua, di cua
x hoi » o hoi, x ho, 5 hoi

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

• Performing a fuzzy search may retrieve terms with similar spellings: tu sach ky nang ((x hoi) or (xa hoi)). hieu ((vien cac) OR (vien da)) tu va dan vi cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: tu sach ky nang ((x hoi) OR (xa hoi)). hieu ((vien cac) OR (vien da)) tu va dan vi cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "tu sach ky nang ((x hoi) or (xa hoi)). hieu ((vien cac) OR (vien da)) tu va dan vi cua tre".
• Adding a wildcard symbol may retrieve word variants: tu sach ky nang ((x hoi) or (xa hoi)). hieu ((vien cac) OR (vien da)) tu va dan vi cua tre*.