SSIS: script task on Visual studio 2013 can not open since Visual Studio 2015 was installed
I have VS 2012, 2013, 2015 installed and faced the same issue.
After looking in the Event viewer it was apparent that it was 2015 VSTA causing the issue.

This solved it for me without re-installing everything....

Uninstalling: 


Microsoft Visual Studio Tools For Applications 2015 
Microsoft Visual Studio Tools For Applications 2015 Language Support


Repairing:


Microsoft Visual Studio Tools For Applications 2012