I was testing some MySQL puppet modules on my Vagrant box earlier this week and one of them required augeas.
I kept running into "Could not find a default provider for augeas", however all the appropriate augeas , augeas-lib and ruby-augeas packages were installed. I inspected the different ruby directories and the files were perfectly in /usr/lib/ruby/site_ruby/1.8 where I expected them.

With all the files seemd to be in the right place, my next option was to strace a small ruby script that included augeas, guess what that showed ..

stat64("/opt/ruby/lib/ruby/site_ruby/1.8/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/site_ruby/1.8/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/site_ruby/1.8/i686-linux/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/site_ruby/1.8/i686-linux/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/site_ruby/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/site_ruby/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/vendor_ruby/1.8/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/vendor_ruby/1.8/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/vendor_ruby/1.8/i686-linux/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/vendor_ruby/1.8/i686-linux/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/vendor_ruby/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/vendor_ruby/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/1.8/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/1.8/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/1.8/i686-linux/augeas.rb", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("/opt/ruby/lib/ruby/1.8/i686-linux/augeas.so", 0xbfd2af1c) = -1 ENOENT (No such file or directory)
stat64("./augeas.rb", 0xbfd2af1c)       = -1 ENOENT (No such file or directory)
stat64("./augeas.so", 0xbfd2af1c)       = -1 ENOENT (No such file or directory)

Indeed ... vagrant throws the default ruby to /opt/ruby .. and obviously there were no ruby-augeas files in there.