Code:

$ echo foo > file
$ cat file
foo
$ ls -i file
472445 file
$ # vim 7.0 opens file in a different terminal, showing just one line, "foo"
$ echo bar >> file
$ cat file
foo
bar
$ ls -i file
472445 file
$ # vim :wq in the other  (after it warns that the file has changed)
$ cat file
foo
$ ls -i file
472449 file

A syscall trace of vim 7.0 reveals the source of the new file with the new inode number:

Code:

rename("file", "file~") = 0
open("file", O_WRONLY|O_CREAT|O_TRUNC, 0644) = 3
write(3, "foo\n", 4)
...
close(3)
...
unlink("file~") = 0

Before writing, this version of vim is using the rename system call to move the original file to file~. It then uses the open syscall with the O_CREAT flag to create a different file. It then writes the contents of the buffer to this new file before unlinking the backup copy (which is the original file and points to the original inode).

Your example does not contradict anything. It merely demonstrates that a different sequence of calls is being made.

Regards,
Alister