Are you sending the two vertices to the same location, then calculating the deltas separately for each, or are you calculating the deltas for one and then applying those to both vertices?  How are you setting it up?  If you're assuming both weld vertices will be in the same place when the calculations are handled, maybe that assumption is the problem? 

===========================sigline======================================================

Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.