Perhaps you should try some spelling variations:
van thiet » va thiet, dan thiet, can thiet
vien cac » vien bach, vien dai, vien da
tinh voi » trinh voi, tinh doi, binh voi
tinh vi » trinh vi, tinh va, dinh vi
su sach » so sach, vu sach
y nang » ky nang, ly nang, v nang
bac va » ban va, bac ha, bao va
ve cac » va cac
x hoi » xa hoi, o hoi, x ho

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. hieu ((ve cac) or (vien cac)) bac va ((tinh vi) OR (tinh voi)) van thiet~.
• If you are trying to use Boolean operators, they must be ALL CAPS: su sach y nang x hoi. hieu ((ve cac) OR (vien cac)) bac va ((tinh vi) OR (tinh voi)) van thiet.
• The words AND, OR and NOT may confuse the search; try adding quotes: "su sach y nang x hoi. hieu ((ve cac) or (vien cac)) bac va ((tinh vi) OR (tinh voi)) van thiet".
• Adding a wildcard symbol may retrieve word variants: su sach y nang x hoi. hieu ((ve cac) or (vien cac)) bac va ((tinh vi) OR (tinh voi)) van thiet*.