Vegard law applies to lattice constants of semiconductor alloys. lf your alloy material is in the form Ax B(1-x) then the formula for bandgap is,

Eg(AB) = x Eg(A) + (1-x) Eg(B) + x(1-x) Eb

Eg(AB) - Energy gap of the graded structure (mixture of A and B)

Eg(A) - Energy gap of A

Eg(B) - Energy gap of B

Eb -bowing parameter (this parameter is usually determined empirically for a system. If your CdSe/PbS structure is already studied then you may find the value of this parameter in literature

x - percentage of A in the alloy

Please have in mind that the above formula applies only to graded heterojunctions. If your junction is chemically abrupt (that is if the composition changes from CdSe to PbS abruptly at the junction) then there will be an abrupt band bending at the junction. Band bending can only be calculated after estimating the work function and electron affinities of the compounds involved. Depending upon the relative values of these parameters, the band structure can be staggered (TYPE II), straddled (TYPE I)or broken (TYPE III). To establish the band structure of such kind of abrupt hetero junctions, usually Anderson's model is used.

As far as the photovoltaic applications are concerned, the optimum bandgap for solar cell active material is from 1 eV to 2 eV where most of the terrestrial solar energy is concentrated. CdSe has a bandgap of 1.7 eV and hence comes in this range. However, the commercialization of cadmium and lead based devices has been less drastic due to their toxicity.

You are talking about multilayer structures. Therefore Vegard Law does not apply as far as you dont have pseudobinary CdXPb1_xS alloys. The optical behaviour has to be modelled with another model that consider the number of layers, refraction index and so on. There are many references on modelling these systems, for example for interferential mirrors. About PV applications, I do not believe that cdse/pbse multilayers woould have pv response at all since you have n-p-n-p junctions, try to see it as a set of diodes connected in  series.. supposing you have single heterojunctions you would possibly can build a pv device if band alignment is correct, try to draw a band diagram of your proposed device. Consider also that PbS has its band gap in the IR range, thats why it is usually employed in ir detectors. 

For a ternary compound semiconductor, you may want to find out the bowing parameter (b) in the literature to fit your Vegard's law equation first. That will give you an estimation of the bandgap. But keep in mind, the bowing parameter is not necessarily a constant number, it may vary depends on the elements ratio in the compound.

It's better to determine the bandgap experimentally. You can measure the absorption spectrum of your sample. 

I agree with Dr. Caballero-Briones,

consider  2 semiconductor A and B and the semiconductor alloy AxB1-x.

EgA the energy gap of semi. A

EgB the energy gap of semi. B

The energy gap of the alloy is given by:

Eg(x)= x EgA + (1-x) EgB - x(1-x)Eb

where Eb is called bowing parameter or bowing energy.

the answer in details in the attachment file.