I am testing vNext in TFS 2015. I have not found a way to trigger builds in parallel. Is that not supported in TFS?
I have found this:
http://www.jaylee.org/
There is currently no equivalent of a drag and drop parallel task. However you can spin up as many build processes as you like using PowerShell. Each of the built in tasks are just PowerShell commands and PowerShell also supports workflow tasks.