How to open correctly project as sub-project

Simple question:

I have project with serveral services inside, each in separate directory, like:
- backend-php
- frontend
etc.

in backend-php I have standard structure:
- public
- src
- tests
- vendor
- composer.json


When I open only backend-php as project everythink is fine. 
But when I open main directory PHPStorm cannot recognise:
- vendor classes telling that they are not exist - Undefined class
- my src dir, telling that PSR-0/4 is not correct setup in composer.json or I should move class to other place

The second problem I solved with File→Settings→Directories by adding prefix, but with vendor I couldn't find any solution.