String theory has a landscape of possible universes. Brane cosmology is related to string theory because it needs extra dimensions, but are the possible different universes of brane cosmology embedded in string theory landscape? Are they part of it?
These are two different things.
The string theory landscape is the collection of all possible vacua. Polchinski in String Theory, Vol I, writes:
The main problem is that whilst there is a unique theory there are an enormous number of classical soultions ... upon quantisation each of these is a possible ground state, ie vacuum, for the theory and the physics is different in each.
DBranes are extended objects on which open strings can end. They range from D(-1), an instanton, D0 a D-particle to D25 a space filling brane. A stack of branes induces a gauge theory and these are then used to build the standard model of particle physics.