First Paralle queries are simply the allowance of more than one connection from the Tableau Server to a specific data source at a time. These limits can be set and modified as outlined here: Configuring Parallel Queries in Tableau Desktop | Tableau Software
I will do my best to provide as many resources about the above:
For data engine vectorization I found:
For Parallel aggregation:
A little more information on external query caching can be found here:
Finally some additinoal information query fusion can be found here:
Hope these resources help!