SQL Server 2012 Business Intelligence licensing
No, you do not have to buy a VS licence to use SSDT or SSDT-BI, you have the licence for the services (IS, AS, RS) if you have a SQL Server licence. SSDT and SSDT-BI are just tools to use those services.

You can freely download SSDT and SSDT-BI from Microsoft:


SSDT: http://msdn.microsoft.com/en-us/data/hh297027 (for SQL Server Database projects)
SSDT-BI: https://www.microsoft.com/en-us/download/details.aspx?id=36843 (for SSIS, SSAS and SSRS projects)


Edit It is not required to preinstall any kind of VS, the will install the VS Shell if it is not installed.

Note You can install BIDS/SSDT-BI using the SQL Server installer, but it contains the version which was available when the installer built (mostly hardly outdated).