I just can't figure out Project Plugin in VIM

In order to configure the project plugin, you must edit the .vimprojects file. An example is given in the help

:help project-example

As you will see, you have to write something like this

Name_of_project = path/to/your/project CD=. flags=flags_you_want{
  file1
  file2
  ...

  Name_of_Subcategory = path/to/your/subcategory filter=files_you_want{
   subcategory_file1
   subcategory_file2
   ...
  }
}

Now you can call your project:

:Project

This will open a vertical window with the file you've just configured. Click on any file you added to open it.

For the flags and the filter, everything is explained in the help file. There are others subtleties you'll have to discover by yourself. I hope this will help you.

You can also have the plugin generate the file for you: invoke :Project in normal mode, and then hit \C which will prompt you for the name of the project, the directory of the project (i.e., ~/Projects/Foobar if ~/Projects/Foobar contains all your code, et cetera), the cd parameter (always the same as the directory, for me) and a standard file filter (*.cpp or what have you).

You can then write this projects file out with :w filename and load it later on.