How can I simply monitoring bandwidth

I have low-end servers for download-upload something. I need to monitoring bandwidth. I know some software for this, but I can't use heavy software on my servers.

So that's why I have an idea: using vnstat and write to csv files average bandwidth.

Vnstat --dumpdb good solution maybe, but not usual for me.

I just need hourly, daily and maybe monthly average bandwidth like this:

hour.csv:

hour,avgdown,avgup
00,123123,1423432 
01,234144,234234
02,234135,245234
...

day.csv:

day,avgdown,avgup
01,...
02,...
03,...

month.csv:

month,avgdown,avgup
01,...
02,...
...

Any software suggestion for this? My servers: 64-128mb ram, just 1 core :).