First of all, as I'm sure you know, Vioxx is a good anti-inflammatory and will probably help with both PF and arthritis, even though it was prescribed for your arthritis. I took Relafen for awhile for arthritis and the pain of PF all but disappeared. However, the disappearance of pain was only as long as the medicational dose -- I didn't want to stay on meds forever so I stoppped -- and the pain returned.
I do find that the PF pain is different from arthritis pain. Arthritis strikes me largely in all the moving joints of my toes and also around my ankles. PF nabs me in the heel, arch and lower metatarsal area. The arthritis is definitely a more 'overall' pain. Sometimes I am pain free, and sometimes they both act up at once -- then it's a little hard to sort them out. Overall for me anyway, the arthritis is much more of a general achiness and stiffness, and the PF is more acute and deliberate pain in identifiable places. Chances are, arthritis cannot be causing heel and arch pain, although it can cause lots of other generalized pain in your foot that you might suspect is PF. Hope this helps.