Problems with Materials/Maps in Viewports (VRay Bug?)

[NoneCG]

Let me share this problem that is driving me nuts these days.

 

I am using 3dsMax2014 & Vray 3.00.03

 

I have this scene which is using a Multi/Sub-Object material for each building block, with each texture/material showing ok on each material ID of the object:

 

 

Now I wanted to add a "dirt effect" to the material using VrayBlendMtl, and then the multiple bitmaps does not display in viewports (just one, regardless of material IDs):

 

 

I tried using other similar kind of materials (standard Blend, Shell, Shellac, etc) and the result is always the same: after mixing the Multi/Sub-Object material with other materials (either max standards or vray's) the ability to show several textures on the viewports is lost, and trying to activate each map just shows it for the entire object without using the proper Material ID's of each face:

 

 

This is a scene with 8 full blocks with several dozens of buildings, hundreds of materials/maps and 60+ Millions of polys... so this problem makes the scene a lot less understandable in the viewports when working with it.

 

Any ideas how to fix this?