Wolfgang Strasser points out that SSRS log files are in a new directory structure for vNext:
The log files can be found in the Logfiles directory (it was the same directory also for the older versions). In SSRS vNext there more different log files..
The logging information seems to be splitted into multiple log files – if you want for example dig into the Power BI on-premises logging I propose to have a look at the RSPower*.log files.
This happens every once in a while, so it’s good to know when the log files move somewhere else.