Should I use a vapor barrier when repairing a portion of the wall?
This is a really interesting question.  Given that you are only doing some of the wall I would say NO.

First I could see the point of doing it.  You put up a vapor barrier and if water gets in the wall it leaves via condensation and doesn't hit the back of the drywall.  Given that you have proper airflow in the wall cavity.  This would certainly be a pro.

The con is that if you get water from above and it trickles down, it will get trapped between the vapor barrier and drywall.  This would cause issues with even a small amount of moisture.  So here I think the con wins.  Also if I have moisture in my walls I would rather just see it faster, not become a bigger issue.  Replacing 2.5 feet of drywall is the optimal issue to have right?