The updates made by background process are not lost, the question is what happens when you are about to do "save & quit" - you will be prompted with message that the original file has changed : do you want to save anyway ?
The other thing is when someone else is trying to vi the same file opened by you - he will get a message :
Quote:
E325: ATTENTION
Found a swap file by the name ".temp.txt.swp"
owned by: sysgate dated: Wed Apr 25 17:29:23 2007
file name: ~sysgate/temp.txt
modified: no
user name: sysgate host name: $myhost
process ID: 30240 (still running)
While opening file "temp.txt"
dated: Wed Apr 25 17:27:07 2007
(1) Another program may be editing the same file.
If this is the case, be careful not to end up with two
different instances of the same file when making changes.
Quit, or continue with caution.
(2) An edit session for this file crashed.
If this is the case, use ":recover" or "vim -r temp.txt"
to recover the changes (see ":help recovery").
If you did this already, delete the swap file ".temp.txt.swp"
to avoid this message.
Swap file ".temp.txt.swp" already exists!
[O]pen Read-Only, (E)dit anyway, (R)ecover, (Q)uit, (A)bort:
but, I suppose you mean large log files, so the solution will vary depending on what exactly you want to do.