How did these end up on Roshar?

[Modal Seoul he/him]

Anybody who has read Words of Radiance knows that both Vasher and Nightblood make appearances. Vasher is now Zahel, a swordsmaster who has a harder time coming up with metaphors than David. In his interlude, it says "there hadn't been one in years", referring to Nightblood's voice in his head. How does he get separated from Nightblood, and how did he get on Roshar? And how does Nale acquire Nightblood? (I know these questions are probably RAFO, but I wanted to know some thoughts) 

[scyron he/him]

My friend went to a signing and asked how Vasher got to Roshar. Brandon said that he worldhops in the same way Hoid does. I assume this just means that he jumps into shardpools and travels through the cognitive realm since we see Hoid do that in SH. Rock also recounts a story of seeing Hoid  jump out of one in the Horneater peaks. I'm pretty sure that is the only currently known method of worldhopping, so Vasher worldhops the same way everyone else does. Unfortunately, I can't provide a source because I don't think its an official WoB.

[Nashan’Elin he/him]

Just btw, could you change the title of this post? I'm pretty sure Words of Radiance is still under the spoiler-policy, so it's best not to have such blatant spoilers in the title. 

[Chaos he/him]

2 hours ago, Nashan'Elin said:

Just btw, could you change the title of this post? I'm pretty sure Words of Radiance is still under the spoiler-policy, so it's best not to have such blatant spoilers in the title. 

It isn't (things under strict spoilers have spoiler boards), but people should generally not put colossal things like this in the topic title regardless, in my opinion. Spoilers are allowed here, but this is a little on the nose.

[dyring]

He´s always been a scholar, so he would be curious. Why he stayed on Roshar is actually rather easy to figure. Easy invetiture.

On Nalthis he need to eat a breath every week, and they are hard to get. On roshar, just a gem and some stormlight, wich is easy to renew.

[Weltall]

We have multiple WoBs that Vasher and Shashara were aware of the existence of Shardblades and intentionally set out to create one of their own, using a different magic system. So Vasher knew about Roshar to some extent before he travelled there and it's part of why he ends up going there (it being easier/less distasteful to obtain Stormlight to live on than Breath).

We also know that Vasher and Nightblood came to Roshar together and Brandon has described them as 'having a falling out'. We might learn about this in Oathbringer, though Brandon had an earlier comment about planning on putting some hints in Warbreaker's sequel so he might want to get that out before giving us the explicit reasoning.

[+Extesian he/him]

We don't know how Vasher and Nightblood got separated, or how Nale has Nightblood, but we know this

Spoiler

RHANDRIC

Is Vasher trying to find Nightblood?

BRANDON SANDERSON

Vasher misses Nightblood and feels responsible for him.