Any consideration from you folks to eliminate the parallel sixths altogether in the chromatic descent (i.e. moving only the 4th finger) and only using them at the very end when the index arrives at the B? Like so:
That might alleviate the unwanted vibrato even more.
This was a change to the original that was suggested to Villa-Lobos by Abel Carlevaro, and one which he (allegedly) gladly accepted.
I also note that a number of pros, albeit not all of them, have adopted this change in that particular section.