Perhaps you should try some spelling variations:
van thiet » va thiet, dan thiet, can thiet
su sach » so sach, vu sach
hieu ve » hieu va, hieu van
tinh va » tich va, trinh van, trinh vat
y nang » ky nang, ly nang, v nang
ve cac » va cac, vien cac
bac va » ban va, bac ha, tac va
bao va » bao ve, tao va, ban va
x hoi » xa hoi, o hoi, 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: su sach y nang ((x hoi) or (x ho)). hieu ve cac ((bac va) OR (bao va)) tinh va van thiet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach y nang ((x hoi) OR (x ho)). hieu ve cac ((bac va) OR (bao va)) tinh va van thiet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "su sach y nang ((x hoi) or (x ho)). hieu ve cac ((bac va) OR (bao va)) tinh va van thiet".
• Adding a wildcard symbol may retrieve word variants: su sach y nang ((x hoi) or (x ho)). hieu ve cac ((bac va) OR (bao va)) tinh va van thiet*.