You need to change following files for a different vendor directory location:

1. artisan

2. public/index.php

3. composer.json

For example, wherever you find following (in case of artisan file):

require __DIR__.'/vendor/autoload.php';

replace with:

require __DIR__.'/php_modules/vendor/autoload.php';

Wherever you find following (in case of public/index.php):

require __DIR__.'/../vendor/autoload.php';

replace with:

require __DIR__.'/../php_modules/vendor/autoload.php';

In composer.json file, look for following fields:

1. autoload -> exclude-from-classmap

2. scripts -> test

3. config -> vendor-dir

Check these above 3 fields in composer.json file and correct the values appropriately.

Thanks (and waiting for more replies/suggestions)