VSFTPD, and any other FTP solution is going to upload / chroot to the users home directory, so if, when you created a user, you set their home directory to /home/username, then that's the directory that will be used by vsftpd.
You can use useradd -d to set the home directory for new users, i.e.
useradd -d /path/to/upload/folder username
So if your username was batric, for example, and you wanted the user to upload to /var/www or /var/www/batric, then you'd use one of the following:
useradd -d /var/www batric
- OR -
useradd -d /var/www/batric batric
You can use usermod to change the directory of an existing user.
usermod -d /var/www batric
usermod -d /path/to/any/directory batric
usermod -d /home/batric