Something is messed in V-Ray Options or Material Editor settings. Save your settings first, then go back to the V-Ray default settings and try to render with Override Materials checked and unchecked then. If there is a problem in both you will have to introduce your settings one by one to see which option causes the problem. There is no way around it, I guess.

You have to use TexMix material in the Diffuse slot. Choose Color A slot to load the first texture and Color B slot to load the second texture. Blend them using Color Mix with the alpha texture loaded in Mix map slot. That's it.