Yes, you would need an additional SQL Server license for SSRS if it is installed on a separate VM. When you install SSRS on a separate VM, that instance is treated as a new SQL Server installation from a licensing perspective. This requires a separate license for the new VM, even if it is only used for SSRS.
Here you will find Brent Ozar answering that same question you have. Read the answers to Curt and Adrian, provided by Brent.
To be certain about having to pay licensing or not, please talk to a Microsoft Licensing Specialist. Please call 1-800-426-9400, Monday to Friday, 6:00 A.M. to 6:00 P.M. (Pacific Time) to directly speak to a Microsoft licensing specialist. For international customers, please use the https://www.microsoft.com/en-us/licensing/worldwide to find contact information. Or you could get help from Volume Licensing Service Center support.