I have a simple scene setup, the cube has 5x5x5 segments, which is important for the next steps.

Apply a simple glass material to the cube. (Uncheck Diffuse - Check Refraction : IOR 1.06, Check Specular Layer 1 and uncheck Trace Specular)

Now select a portion of the model and choose "Set Vertex Weight" from the selection menu.

Your "Vertex Weight" selection is now visible in the Editor view and the 'OM' (object manager)

Deselect the Vertex Map in the OM and create a second selection and a second Vertex Map (Once a weight is determined it`s called a 'Vertex Map')

You should have 2 vertex maps. It`s helpful to name them to differentiate later on.

Make 2 additional copies of your Glass material and again name accordingly (ensure 'base' is already applied material).

* In the 2 material, Choose the Vertex Map option from the effects Rollout.

Drag the first Vertex Map in to the 'Vertex Map' Slot

In the Refraction Options of this material change IOR #. You may change any of the setting for various effect.

Material 2 now completed. Follow the same steps from "*" again for material '3rd'. Using another slightly different IOR.

When completed add materials 2 and 3 as addition to base material .

It`s wise to Mix your 3 materials in Vray, but interesting effect can be achieved by not following this rule.

The example has varying colors in refraction channel.

Small displacement.

Diffuse / Transparent blend

One solid and two transparent.

The Vertex Map weight does not need to be 100 %. Choose 80-95 % for an even blend between the two.
The vertex mapping technique can be applied for any number blend effects.

Experiment! It`s not always dependent on the "Material Weight" settings in Vray material. The vertex map can have very interesting effects on other channels.