[ubuntu] Grub and vnstat puzzles

No longer a show-stopper, but two possibly coected puzzles.

Background

I have a triple-boot system, all Ubuntu.
12.04 on sdb1
10.04 on sda3 (almost never used)
8.04 on sda1 (never used)

Yesterday I did some work using 12.04 (Precise), including dealing with a few emails and some stuff with LibreOffice spreadsheets and then shut down normally.

First puzzle

Today when I logged on, grub did not offer me 12.04 at all, just the extant 10.04 and 8.04 keels as options.
Nothing was amiss on sdb1 as seen from 10.04 (Lucid), but I did notice by looking in the file properties that /var/lib/vmlinuz claimed it had been accessed around the time I had signed out.

Problem with boot was eventually cured by editing the boot sequence in grub (it had not been looking at sdb1; somehow another drive - one not capable of booting from - had been substituted).
How could this have happened without my being aware of doing something out of the ordinary? Normally I leave anything to do with the OS well alone.

Second puzzle

I use vnstat routinely to keep an eye on network traffic and soon before I shut down yesterday noticed that it was not reporting any data for yesterday.
vnstat uses a file owned by root in /var/lib/vnstat as a store for the data (gathered, I understand by/through the keel).
When using Lucid, vnstat was working normally - in other words nothing was preventing the data being collected (different physical file, of course).
Once I got into Precise earlier today, vnstat was still not reporting traffic, so I pretended it was new and tried vnstat -u -i eth0 which told me it could not read the file (fair enough, owned by root) nor write to it but, despite this, subsequently data is being collected and reported. Why not before? Is this coected with the first puzzle?

So...If anyone can shed light on these puzzles, my curiosity would be satisfied and the knowledge may be of use to someone facing the same situation.
As I say, I'm up and ruing as normal again, so it's just a matter of interest for me, nothing more.