Unicorn Rails stack + Vagrant not serving some assets - ruby-on-rails

Question

I'm using a Rails Stack with nginx + unicorn + rails meant for a production server, but I'm staging it under Vagrant for testing purposes. While doing this, I encountered a strange behavior of the rails application, where very often one or other asset wasn't being served, i.e. application.css isn't being served and therefore the whole page is displayed without any styles applied to it.
I've googled the problem and found that Vagrant's FS driver isn't completely implemented and that would bring some problems while using Apache (haven't found any mentions to nginx). The solution to this problem was to disable sendfile by adding sendfile off; to the configuration file. And... it didn't work.
Further, I went through the logs (Rails, unicorn and nginx) and found that when the file isn't served, there isn't any mention to it in any of the logs. This brought me to the question that the problem can be in the mechanism used by Vagrant to share the rails app folder through the VM. As mentioned in vagrant's website, Vagrant uses Virtual Box's Shared Folders, which is quite slow comparing to other alternatives (as shown here), and the workaround is to set up NFS Shared Folders. So, I decided to give NFS a try and the result was... the same. Unfortunately, some assets are kept from being served.
Does anyone have any ideas on this? I've searched for quite a while but haven't found any pointers additional to those I described here.
I'm using:
Mac OS X 10.6.8 + rbenv (to develop)
Vagrant + nginx + rbenv + unicorn + bundler (to stage)
unicorn.rb
rails_env = ENV['RAILS_ENV'] || 'production'
app_directory = File.expand_path(File.join(File.dirname(__FILE__), ".."))
worker_processes 4
working_directory app_directory
listen "/tmp/appname.sock", :backlog => 64
#listen "#{app_directory}/tmp/sockets/appname.sock", :backlog => 64
timeout 30
pid "#{app_directory}/tmp/pids/unicorn.pid"
stderr_path "#{app_directory}/log/unicorn.stderr.log"
stdout_path "#{app_directory}/log/unicorn.stdout.log"
preload_app true
GC.respond_to?(:copy_on_write_friendly=) and
GC.copy_on_write_friendly = true
before_fork do |server, worker|
defined?(ActiveRecord::Base) and
ActiveRecord::Base.connection.disconnect!
end
after_fork do |server, worker|
defined?(ActiveRecord::Base) and
ActiveRecord::Base.establish_connection
end
/etc/nginx/sites-enabled/appname
upstream unicorn_server {
server unix:/tmp/appname.sock fail_timeout=0;
}
server {
listen 80;
client_max_body_size 4G;
server_name _;
keepalive_timeout 5;
# Location of our static files
root /home/appname/www/current/public;
location / {
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_redirect off;
if (!-f $request_filename) {
proxy_pass http://unicorn_server;
break;
}
}
error_page 500 502 503 504 /500.html;
location = /500.html {
root /home/hemauto/www/current/public;
}
}