Vim config problem in jessie

[Frank Miles <fpm@u.washington.edu>]

I'm having problems setting the vim configuration - 
and having it mean something - in a fairly new computer.
One simple example is the tab stops.  Since I'm the
only direct user of this machine, I've simply edited
/etc/vim/vimrc, where I have the line:
	set tabstop=4
In addition, I've appended the line
	set ts=4
to some of the 'plugin' files, for example:
	/usr/share/vim/vim74/syntax/python.vim
{the normal ending line with ts=8 has been commented-out;
otherwise the plugin file is unaltered from the Debian original}.

However when I open a python file, it's obvious that
8-space tabs are being used.  Doing a set: shows that
	softtabstop=4
But there is no
	tabstop=4
whereas opening a 'C' file will achieve the latter (and
'proper' spacing.

The file is recognized as python - the keywords are highlighted
as expected.  Manually entering the set ts=4 sets things right.

I've tried creating a ~/.vim/vimrc to no avail.  I've not had
any problem with previous incarnations of Debian.

One has to suspect that some config file is overriding mine.
A quick examination of the installed vim packages using
	dpkg -L ...
has turned up nothing other than links to /etc/vim/vimrc

Anyone have any hints regarding the source of this annoyance?
TIA!
	-F