Exactly who knows who in the Cosmere is still an open question, but based on what we know, it's likely that they each other, but they aren't working together.
We know that Hoid has been active on Roshar for a while, given how many people know him as Wit. Similarly, Vashar is an established swordsman in the royal court, trusted with the training of princes, so he must have been around for a while. This makes it unlikely that Vasher was one of the people sent after Hoid, since they appear to have only recently started that search.
However, there's also no indication that he's been doing anything to help Hoid. Rather, he seems to be on Roshar simply because it's easier to stay alive:
It’s pretty obvious that the way that the Breath’s working, the reason he moved is because it’s easier to get Stormlight than Breaths, and Stormlight can fuel being a Returned like him. src
There's a pretty strong implication that this group of worldhoppers are a small community, and so they probably all know each other, or at least know of each other. Hoid would most likely recognize a Returned using Stormlight instead of Breath to stay alive, for example. But beyond that, there's no indication that the two of them have any deeper connection.