This page gives information on the VRayProxy node and how to use it.
VRayProxy imports geometry from an external mesh at render time only. The geometry is not present in the scene, and does not use up any resources. This allows the rendering of scenes with many millions of faces, more than Maya itself can handle.
The V-Ray proxy in Maya creates a VRayMesh node which loads the geometry from a file on disk. The node can load alembic (.abc) and V-Ray proxy (.vrmesh) files. See Import V-Ray Proxy.
If you wish to import a Maya mesh through a VRayProxy object, the mesh must first be exported to a .vrmesh file either from the Maya user interface, or through a MEL command. See Export Mesh to a File for details.
The .vrmesh File Format
Meshes are exported to a special .vrmesh file format. It contains all geometric information for a mesh---vertices and face topology as well as texture channels, face material IDs, smoothing groups, normals---in short, everything that is needed to render the mesh. In addition, the mesh is preprocessed and subdivided into chunks for easier access. The file also contains a simplified version of the mesh used for preview purposes in the viewports. More information about the .vrmesh file format is available in the V-Ray SDK documentation that comes with the V-Ray for Maya installation.
It is important to realize that the mesh is in a "ready to render" format. No further modifications to the mesh are expected. You can't apply modifiers to the mesh, or animate it in any way other than animating the position/orientation of the proxy object. There is no way to recover the original mesh from a .vrmesh file (this can be done in principle, but is currently not supported). Therefore, if you plan on doing modifications to the mesh, you should keep it in a Maya file (which may be different from the file that gets rendered in the end).
The VRayMesh node can also load and render Alembic files (.abc). The supported geometry types are polygonal meshes (without subdivision surfaces), spline curves and particles.
V-Ray will recognize velocity information for motion blur if it is stored either as standard Alembic velocity, or as a 3D vector array property called arbGeom.v, arbGeom.velocities or arbGeom.velocity. Additional Alembic color sets are imported as mapping channels with indices greater than 0 (i.e. 1, 2 etc). It's also possible to use such a color set for velocity. The VRayMesh has some additional parameters related to Alembic files. See the Alembic Proxy Parameters section of the Import V-Ray Proxy page for more details.
The resulting .vrmesh files can be rendered outside of Maya - for example, by the standalone version of V-Ray, or V-Ray for 3ds Max.
You can convert .ply, .obj, .geo and .bin files to .vrmesh files with the help of the ply2vrmesh converter tool.
The V-Ray SDK has methods for reading and writing of .vrmesh files.
Official Alembic file format home page: http://www.alembic.io/