Factor VIII (factor 8, anti-hemophilic factor) is present in vascular
endothelium and, and along with CD34 is used as an immunohistochemical
marker for small blood vessels. Factor VIIIa is the activated form of
this clotting factor.
von Willebrand factor is an entirely different molecule, also in
vascular endothelium, and is bound to circulating factor VIII.
There are fairly clear accounts of these factors in Wikipedia.
Histonet mailing list