I tried to deal with WMF files in the past and could never really get them to behave either. I don't know if more recent versions of Word can accept other vector formats or not. The stock solution was usually just to create high-res bitmaps of the vector work for the person to input into MS Word (which, of course, made it impossible to make slim PDF files for email purposes...)

http://www.creativetechs.com/iq/create_letterhead_templates_in_ms_word.html

https://listserv.heanet.ie/cgi-bin/wa?A2=ind0511&L=typo-l&T=0&P=11403

Thanks. I went with a psd.

I would use a high res PNG (300 dpi or higher). PNG is supported well by the Microsoft Office line and it also supports transparency with little to no artifacts.