No.
Two is the minimum number of vectors that will sum to zero.
The minimum number of vectors with unequal magnitudes whose vector sum can be zero is two. These vectors must have magnitudes and directions that cancel out when added together to result in a zero vector sum.
Two vectors: no. Three vectors: yes.
No.
Yes. Any number of vectors, two or more, can result in zero, if their magnitudes and directions are just right. One vector can result in zero only if its magnitude is zero.
equal and opposite
No.
Yes, two vectors with different magnitudes can be combined to give a zero resultant if they are in opposite directions. However, it is not possible for three vectors with different magnitudes to give a zero resultant because they must have specific magnitudes and directions to cancel each other out completely.
Yes. Vectors contain both magnitude and direction. Graphically three vectors of equal magnitude added together with a zero sum would be an equilateral triangle.
It is certain that two vectors of different magnitudes cannot yield a zero resultant force.
Vectors that sum to zero are coplanar and coplanar vectors sum to zero.
-- The minimum magnitude that can result from the combination of two vectors is the difference between their magnitudes. If their magnitudes are different, then they can't combine to produce zero. -- But three or more vectors with different magnitudes can combine to produce a zero magnitude.