To reproduce:
0) Don't have VS2017
1) Run Unity Download Assistant
2) Install VS as one of the components
3) After installation finishes, run Unity Download Assistant (version doesn't matter)

Expected: Assistant runs without prompts
Actual: This is prompted right before anything http://i.imgur.com/37OiEvv.png

Note: Running VS once to make sure it works properly (and setting up license) doesn't help

Tested:
Installed VS with 2017.3.0a6, 2017.1.1f1, 5.6.3p2 assistant
After that ran these assistants (and installed all the corresponding Unities): 5.6.3p2, 2017.1.0f3, 2017.1.0p5, 2017.1.1f1, 2017.2.0b9, 2017.3.0a6

Tested on Microsoft Windows [Version 10.0.15063]
VS version that was installed: http://i.imgur.com/qYsbkZi.png