Perhaps you should try some spelling variations:
viet sach » viet anh, dien sach, viet tech
hieu viet » sieu viet, hieu viec, lieu viet
sach nuoc » sach duoc, cac nuoc, bach tuoc
vanh dai » van dai, vanh tai, lanh dao
su sach » so sach, vu sach
sach ky » sac ky, sach y, sach ty
nang ma » tang ma, sang ma, mang may
xanh di » danh di, xanh tai
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: su sach ky ((nang ma) or (nang cao)) hoi. hieu viet sach nuoc va ((xanh di) OR (vanh dai)) cua tre~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach ky ((nang ma) OR (nang cao)) hoi. hieu viet sach nuoc va ((xanh di) OR (vanh dai)) cua tre.
• The words AND, OR and NOT may confuse the search; try adding quotes: "su sach ky ((nang ma) or (nang cao)) hoi. hieu viet sach nuoc va ((xanh di) OR (vanh dai)) cua tre".
• Adding a wildcard symbol may retrieve word variants: su sach ky ((nang ma) or (nang cao)) hoi. hieu viet sach nuoc va ((xanh di) OR (vanh dai)) cua tre*.