Your best bet would be to install Outlook on the Vista machine; keeping
Outlook synced on two machines is fairly simple, while keeping Outlook on XP
synced with WM on Vista can be quite a challenge.  The problem is, Outlook
must be installed on the machine doing the transfer; Outlook has to be there
to access the .PST data file.  Ordinarily, you'd transfer from Outlook to OE
on the XP machine, take the OE .DBX files to the Vista machine, and import
them into Windows Mail; this is a one way street, however, since WM won't
readily export to Outlook Express .DBX files (Windows Mail saves everything
as discrete .EML files which would have to be copied, dragged and dropped
back into OE folders on the XP machine, then transferred back to Outlook
with Outlook Express).  If you install Outlook on the Vista machine, syncing
is then a matter of merely copying the .PST file back and forth between the
two machines:

http://www.slipstick.com/outlook/sync.asp
Synchronizing Outlook on Two Machines

Hal

My OEX program (www.oehelp.com/OEX/) lets you import from dbx files and also
from directories of eml files and it works both in XP (OE) and Vista
(WinMail).  It doesn't actually do a synchronization, but you can use those
features to manually synchronize.

steve