To get the vector for spheres:

#declare centerA=<10,5.0,-5.50>;
#declare radiusA=3.1415;
#declare centerB=<1.0,-45.0,15.50>;
#declare radiusB=0.1415;

#declare 1stpoint=  centerA+ radiusA*vnormalize(centerB-centerA);
#declare 2ndpoint=  centerB+ radiusB*vnormalize(centerA-centerB);

Then the vector in question is between 1stpoint and 2ndpoint.
A treatment for "circles"  is the same as that for spheres with coplanar
centers.

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