Fixes issue with vsiten and Verlet buffers

Commit 0336ab2d only fixed part of the vsiten issue in the Verlet
buffer calculation code. Parameters were read from incorrect memory
locations, which could lead to a segmentation fault or incorrect
masses for vsiten particles. It unlikely that this affected results.
Part of #1579.

Change-Id: I76cdb94e34194d2f6d49d98a49486ce1df76d91a