The reason the lower left leg breaks off is that the lower vertices are slightly misaligned. In the image on the left, the lower vertices at the "crotch" appear to be aligned correctly (they appear identical), but using Poser front camera at scale 0.5, with 0.000 rotation for both lower legs, we see that the "crotch" vertices are misaligned (not identical). Since they cannot be easily aligned in Poser, you must go back into your vertex modeller, at 800X or 1600X magnification, and make sure those vertices are absolutely identical in their (x,y,z) coordinates.