The ROP Fetch TOP looks for an output file parameter on the target ROP in order to determine where the ROP writes its files. That's necessary for e.g. caching support, so that TOPs can know what outputs already exist before cooking any of the work items.
When the ROP Fetch isn't able to determine which parameter on the ROP defines the output path(s), it issues a warning. It is just a warning though -- not an error -- and TOPs will still cook the ROP. It just can't providing caching functionality or output file reporting since it doesn't know what file paths are going to be produced.
If this used to work before, and doesn't in newer versions, it sounds like the Vray ROP has changed it's parameter interface and renamed the parameter it uses to define output paths. By default, TOPs checks the ROP node for any of the following file parameters:
vm_picture,
sopoutput,
dopoutput,
lopoutput,
picture,
copoutput,
filename,
usdfile,
file,
output,
outputfilepath,
outputimage,
outfile#.
You can explicitly tell TOPs which parameter on the ROP node defines the output file path by setting the
Output Parm Name on the ROP Fetch TOP, as described in the docs here:
https://www.sidefx.com/docs/houdini/nodes/top/ropfetch.html#customoutput [
www.sidefx.com]
I don't have Vray installed, so I can't check myself. But double check the name of the output parm on the Vray ROP, and set that as the Output Parm Name on the TOP node.