He isn't waiting around he is finishing his training so he will successfully rescue his friend (who isn't going anywhere trapped in carbonite).

The last time Luke rushed into a similar situation he was not fully trained, lost a hand and needed rescuing himself.

It makes sense.

Also you have a double dip on the roller coaster.

A nice slow ascent for the first dip followed by the Rancor and Sarlacc action and then another for the finale (the Emperor scenes, the briefing, the landing on Endor) and then the bike chases and all the fighting in the trees, in space and dueling with lightsabers.

The first Vader scene is basically an introduction to the major problem of the whole episode, the Death Star is being replaced (thus annulling the Rebels one big victory) Vader is still after Luke and the Emperor is coming to town.

Vader and prune face only need to make their next appearance after Luke has rescued his friends and dealt with the smaller problem of Jabba.