A mechanic who works on Vikings should be able to find a screw that works for you. Find out if the Viking stores have a mechanic who comes in (or that you can visit yourself). It would probably be best to bring in both the machine and the foot.
Also, try Googling online for Viking parts. You may be able to find the screw yourself that way (although shipping will probably far exceed the cost of the screw).
Here's a link to an online store's Viking section:
http://www.sewingpartsonline.com/vik...odel-list.aspx
Check your model number to see if you can find the screw you need.