Same for me, tried everything but no matter what, same thing happens =/

I've started again on a regular campaign with the Black Sun and the same thing happens.

This time it's at around week 250, I'm about to assault Hoth but then the Death Star 2 cutscene happens (I haven't built it on Endor yet) and Vader disappears. No explanation why or any mission holocron updates.

If you have started the DS construction mission( after u build byss research facility and u get a mission you have x weeks to get DS2 built) and you dont finish the death star before then u will auto loose vader palpy and piett. Now some weeks before DS2 deadline Vader will go to endor and stay there till the battle so if ur close to it ur not getting vader. Ah and hoth chain gives piett which u can get from the endor mission. So remember once u build byss research facility u gotta get to endor asap and start building DS2 it costs 200 pop and alot of creds.

Thanks for creating a thread about this. I've seen a few reports on the issue but its good to have them all in one place, and with as much detail as possible. I'll pass it on to the team. As lord of pain says this might partly be intentional mechanics, but if so they clearly aren't being communicated well enough.

Thank you General! However it seams pretty common, not only us 3 poor bastards. Seen the same bug in different forums. Here´s my problem in more detail.

When playing the Empire campaign (same bug in both campaigns) If Vader dies he never respawns (mission to save him) and he disappears If I get too many heros, or once the cinematic for the Hoth mission triggers. Everytime, no mater how clean install/save files or nothing.

So, I've spoken to the devs involved with this and Vader disappearing as per the OP is intentional. The idea is that Vader is no longer flying about in his ISD because he is on the death star. If you complete the death star in time and defeat the rebel fleet at Endor you will get him back. If not, he is gone for good as per episode 6.
If you don't build the death star when asked you will fail and lose Vader and Palpatine. As such you will never be able to do the hunt missions either if you haven't done already, so no Thrawn or Piett.
I have passed on that we need to be explaining this better to players, adding some popups or something.
Any issues you've experienced outside of this are most likely separate issues / bugs which i'm happy to address elsewhere.