The 2D Placement Options attributes can also be added to Maya's samplerInfo node. This can be used to output UV passes for a specific object and a specific UV set in case the object has multiple UV sets. To do this, the samplerInfo's U and V outputs can be connected to the R and G texture inputs of the VRayExtraTex render element. Then the UV set to be used is specified in the UV set name field of the 2D Placement Options attributes assigned to the samplerInfo. If needed, multiple samplerInfo nodes with different UV set names specified can be used with multiple VRayExtraTex render elements to output more than one UV passes at the same time.
An alternative and easier way is to use the Sampler Info render element, which includes an UVW Channel field.