Changing the ownership to a designated user will not change the users default home directory specified in passwd file. Anyhow, You may achieve this in different ways. But to make it simpler do this by opening up a terminal;
Firstly make a back-up of your /etc/passwd file doing below;
sudo cp /etc/passwd /etc/passwd.back
Once done, edit the file (either with vi or nano), I prefer nano which is pretty easy;
sudo nano /etc/passwd
Locate the created user. A line should reflect as below assuming the user is andrew;
/var/www/sitename.com & save the file by hitting CTRL+O then enter to save & then CTRL+X to exit. This will direct the user to specified directory. Login as the user and see whether it points to **/var/www/sitename.com by also checking for any permission issues. If permission issue persists, then add the user to the
www-data group by doing below;
useradd -G www-data andrew
EDIT: regarding the comment, I just had a quick search and found some more detail. Well, you may edit vsftpd.conf file located in
sudo nano /etc/vsftpd.conf
un-comment (remove only the # if its there at the beginning) the following line;
Note: Just to be safe, make a back-up of the configuration file before you make any changes to it.
sudo cp /etc/vsftpd.conf /etc/vsftpd.conf.back
Source: How to create a FTP user with specific /dir/ access only & vsftpd.conf on Ubuuntu
Hope it helps!