- Create glass model (box), the right size and thickness. - Lodge at her the VRayDisplacementMod . - Bury / do such a texture falling water. - Lodge of the texture Texmap slot. - To apply for this model conventional glass mat.
For more realism, you can just modify the glass material. For example, make a card for the glossiness of the same texture all of the falling water. If done correctly, there where no drops - will perfectly transparent glass, and where there is - a bit blurry. The same with defyuzom where flat glass, there is a dark color, and where drop - light and foam will effect😉