To find the angle between the two vectors, you need to normalize them (so they have length of 1 unit) by doing this (v is the vector you want to normalize):

float distance = sqrt(v.x*v.x + v.y*v.y + v.z*v.z);
v.x = v.x/distance;
v.y = v.y/distance;
v.z = v.z/distance;
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Then find the dot product of the two normalized vectors (Va and Vb):

float dotProduct = Va.x*Vb.x + Va.y*Vb.y + Va.z*Vb.z;
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This gives you the cosine of the angle, so simple do:

acos(dotProduct);
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This will give you the angle. I think this post was a bit late :S