At later levels (10+ Imperial Floors), the rewards for completing Darth Vader's quests are in the 100k+ range, so Imperial floors are perhaps the most lucrative. To prove the point, a level 6 commercial floor with three 9 stars will generate 50k+ when all items are stocked (which can take 6+ hours, but is fairly passive). By building up the Imperial resources at every chance (at least ten of each one), you can reduce the amount of time needed to complete Vader's quests.
Regarding VIPs, make sure you haven't saved too many. Once you have five saved VIPs, you will reduce the likelihood of getting more; especially of the type you have saved. It's almost like the game doesn't like you having so many saved. I had an Upgrader VIP I was saving for the Emperor, and I didn't see another Upgrader for five days straight(until I used my saved Upgrader). If you use them, they will be replenished. If you save them, it appears the chance you will get more diminishes.