Okay - this would be a bit more complicated, I'm afraid.
Binary installation:
Modify your /etc/apt/sources.list to include the standard Debian repositories, and then install it with:
sudo apt-get install vnstat
Source installation:
Ensure that your machine has the necessary compiler etc.:
sudo apt-get install build-essential
download the
tar.gz file:
wget http://humdi.net/vnstat/vnstat-1.7.tar.gz
extract it:
tar xvf vnstat-1.7.tar.gz
chanfge to the output directory:
cd vnstat*/
configure it:
./configure
compile and link it:
./make
install it:
sudo make install
In either case, you should now be able to run it - but you'd need to read the
man page to configure it to your requirements, and to point it at the
ppp0 interface, or whatever the cellular connection is on your device.
Neil