Maybe someone can correctme, but do you know if either a random story mission popped and vader was taken away for it? Or perhaps maybe IG-88 or someone 'assassinated' him temporarily?

Otherwise it sounds like a bug as to why vader disappeared. As you need both Ozzel and Vader with a fleet for to start the Hoth battle, i believe.

Hi, he just disappeared after the cut scene entering the system before hoth. Maybe I shouldn't started building the Death Star before invaded Hoth. But the Death Star isn't finished yet. So he shouldn't disappear at all? I thought he joined Ozzel on his Star Destroyer and that is why he wasn't playable anymore till I hit Hoth.

Where in the DS II building are you? Because i think at some point after the maw attack vader does disappear for the project...? Or maybe you had to move him to endor at the point i am thinking of?

Its been a hot minute since i played the mod so i can't remember, sorry. Hopefully someone will get back to you with a bit more insight than i have.

I just tested it. I aborted the Death Star and started it again. After 30% the Emperor disappeared. The building site ate Vader and now Palpatine, lol. So don't start to build the Death Star if you want to attack Hoth. I will have to load a save before I started building the DS2 I guess.