You can spawn a cab when you get outside the save house no matter the vehicle density.

In some of the mission (mostly concrete jungle) i realized, i had huge problems spawning a cab (which also can happen on 100 vehicle density). What is the connection?

As it turns out the game spawns very little cabs, if you have stolen cab before. You can get rid of this by video editor warping after you have stolen the cab. The effect of stealing a cab seem to last very long.

If I don't steal the cab in Concrete Jungle, the cab will always spawn on one vehicle density.

This is still in the works, but for now don't steal any cabs anymore in the run and if you do make sure you video editor warp afterwards.