Can you pastebin your entire config file, and ls -al /Users/Dropbox/dat/web.
Is anything shown in error.log?
total 0
drwxr-xr-x@ 10 thanhdat staff 340 Jan 18 21:59 .
drwx------@ 16 thanhdat staff 544 Jan 18 22:04 ..
drwxr-xr-x@ 7 thanhdat staff 238 Jan 18 22:00 bootstrap
drwxr-xr-x@ 10 thanhdat staff 340 Jan 18 22:00 learn
drwxr-xr-x@ 10 thanhdat staff 340 Jan 18 22:01 phpmyadmin
drwxr-xr-x@ 8 thanhdat staff 272 Jan 18 22:00 shop
drwxr-xr-x@ 3 thanhdat staff 102 Jan 18 22:00 tuanvu
drwxr-xr-x@ 100 thanhdat staff 3400 Jan 18 22:01 upload
drwxr-xr-x@ 77 thanhdat staff 2618 Jan 18 22:01 vb3
drwxr-xr-x@ 20 thanhdat staff 680 Jan 18 22:00 xenforo
[Sun Jan 18 22:17:05.622256 2015] [core:error] [pid 981] (13)Permission denied: [client 192.168.1.138:50698] AH00035: access to / denied (filesystem path '/Users/thanhdat/Dropbox/www') because search permissions are missing on a component of the path
[Sun Jan 18 22:17:05.834166 2015] [core:error] [pid 981] (13)Permission denied: [client 192.168.1.138:50698] AH00035: access to /favicon.ico denied (filesystem path '/Users/thanhdat/Dropbox/www') because search permissions are missing on a component of the path
[Sun Jan 18 22:17:08.234748 2015] [core:error] [pid 981] (13)Permission denied: [client 192.168.1.138:50698] AH00035: access to / denied (filesystem path '/Users/thanhdat/Dropbox/www') because search permissions are missing on a component of the path
[Sun Jan 18 22:17:08.332498 2015] [core:error] [pid 981] (13)Permission denied: [client 192.168.1.138:50698] AH00035: access to /favicon.ico denied (filesystem path '/Users/thanhdat/Dropbox/www') because search permissions are missing on a component of the path
Is the error a 403 Forbidden error in the browser? If so, paste your site configuration.
Your first post shows the root directory being at /Users/dat/Dropbox/web
Then you showed the ls -la results from /Users/thanhdat/Dropbox/www
Which is it, web or www?
Also, I'm assuming that you just included a snippet from your config file and that you have the requisite </Directory> tag (and everything in between).
Are you restarting Apache after changing the site config?