I may be grasping at straws here, but I got to thinking of the similarity between the names of the two, and if Vasher could worldhop to Roshar, is it possible that Shalash/Shashara is the same person and started on Roshar and then ended up on Nalthis? If this is a possibility the larger implication is that the Heralds could have gone to other planets as well after abandoning the Oath Pact. Perhaps we've even met some of them.