If v1 and v2 are normalised so that |v1|=|v2|=1, then,
angle = acos(v1ov2)
axis = norm(v1 x v2)
If the vectors are parallel (angle = 0 or 180 degrees) then the length of v1 x v2 will be zero because in(0)=sin(180)=0. In the zero case the axis does not matter and can be anything because there is no rotation round it. In the 180 degree case the axis can be anything at 90 degrees to the vectors so there is a whole range of possible axies.