Well, since I can't delete this post, and maybe someone in the future might find this helpful: I figured it out! Someone messed with the styles editor. Vendor was always part of the BOM in our standard template, but with a width of 0. That seems to confuse Inventor, where it doesn't show it at all, not even in the Parts List Window. Solution: Update column width to something useful in the styles editor, remove the column, click OK. Go back in and re-add the column. Then it works.