.gitignore whitelist on directory and its contents

You can also achieve this with only one .gitignore file (in your project root):

/*
!/.gitignore
!/vendor
/vendor/*
!/vendor/SupplierName

You can use 2 .gitignore files to achieve the desired result:

# vendor/.gitignore
*
!.gitignore
!SupplierName/
!SupplierName/*

# vendor/SupplierName/.gitignore
!*

I tested this with a test repo and seems to work for me in adding files as many levels deep underneath the vendor/SupplierName directory.

$ git add .

$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   modified:   vendor/.gitignore
#   new file:   vendor/SupplierName/.gitignore
#   new file:   vendor/SupplierName/a
#   new file:   vendor/SupplierName/b
#   new file:   vendor/SupplierName/c
#   new file:   vendor/SupplierName/d
#   new file:   vendor/SupplierName/dir1/d
#   new file:   vendor/SupplierName/dir1/dir4/dir5/dir6/dir7/dir8/dir9/dir10/somefile
#   new file:   vendor/SupplierName/dir1/dir4/f1
#   new file:   vendor/SupplierName/dir1/dir4/f2
#   new file:   vendor/SupplierName/dir1/dir4/f3
#   new file:   vendor/SupplierName/dir1/dir4/f4
#   new file:   vendor/SupplierName/dir1/e
#   new file:   vendor/SupplierName/dir1/f
#   new file:   vendor/SupplierName/dir3/dir6/f5
#   new file:   vendor/SupplierName/dir3/dir6/f6
#   new file:   vendor/SupplierName/dir3/dir6/f7
#   new file:   vendor/SupplierName/dir3/dir7/f8
#   new file:   vendor/SupplierName/e
#