This OSL Shader is available in Wall Worm/Wall Worm Pro v3.74+ for 3ds Max 2019. It mimmicks the WorldVertexTransition shader in Source. It will output DiffuseColor, DiffuseAlpha, SpecularColor, SpecularAlpha, BumpColor and BumpAlpha. It also supports seamless_scale.

Note that because OSL does not yet currently support Channel -2 (Vertex Alpha) it means you cannot use the current vertex alpha channel (used by the DX Shader) but must copy it from -2 to the channel you wish to use in the map.