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This does not compile:

   cv::Matx33f M3x3;
   cv::Vec3f v3x1 = M3x3.col(0); // compile error:

with error (on VS2008):

error C2440: 'initializing' : cannot convert from 'cv::Matx<_Tp,m,n>' to 'cv::Vec<_Tp,cn>'
with
[
    _Tp=float,
    m=3,
    n=1
]
and
[
    _Tp=float,
    cn=3
]
No constructor could take the source type, or constructor overload resolution was ambiguous

That's because although Vec3f derived from Matx33f there is no proper constructor that allows this to happen.
So 2 questions:

1. What would be the cleanest way to make this assignment.
2. Why not add such a constructor?
   I guess it would also have to accept this: cv::Vec3f v3x1 = M3x3.row(0);.

This does not compile: compile:

   cv::Matx33f M3x3;
   cv::Vec3f v3x1 = M3x3.col(0); // compile error:

with error (on VS2008):

error C2440: 'initializing' : cannot convert from 'cv::Matx<_Tp,m,n>' to 'cv::Vec<_Tp,cn>'
with
[
    _Tp=float,
    m=3,
    n=1
]
and
[
    _Tp=float,
    cn=3
]
No constructor could take the source type, or constructor overload resolution was ambiguous

That's because although Vec3f derived from Matx33f there is no proper constructor that allows this to happen.
So 2 questions:

1. What would be the cleanest way to make this assignment.
2. Why not add such a constructor?
   I guess it would also have to accept this: cv::Vec3f v3x1 = M3x3.row(0);.

This does not compile:

   cv::Matx33f M3x3;
   cv::Vec3f  v3x1 = M3x3.col(0); // compile error:
error
   cv::Matx31f m3x1 = M3x3.col(0); // this is ok

with error (on VS2008):

error C2440: 'initializing' : cannot convert from 'cv::Matx<_Tp,m,n>' to 'cv::Vec<_Tp,cn>'
with
[
    _Tp=float,
    m=3,
    n=1
]
and
[
    _Tp=float,
    cn=3
]
No constructor could take the source type, or constructor overload resolution was ambiguous

That's because although Vec3f derived from Matx33fMatx31f there is no proper constructor that allows this to happen.
So 2 questions:

1. What would be the cleanest way to make this assignment.
2. Why not add such a constructor?
   I guess it would also have to accept this: cv::Vec3f v3x1 = M3x3.row(0);.