Both of these chains have been traced back through history thousands of years; Viking knit got its name from being found in graves in Scandanavia - some fragments suggest to archaeologists that Vikings and other cultures in ancient Europe would cut this chain into segments and use it as currency!
You'll want to use a wooden drawplate for this. Metal drawplates are for drawing down individual pieces of wire, so they're too small to use for reducing chain. A wooden draw plate will gently coax your wire chain into longer and narrower segments while allowing the wire to glide past.
Albina Manning taught us how to make Viking Knit Chain in her DVD. You'll get to see every step as she makes her own tools, creates the stitch, joins wire, draws the chain through a wooden drawplate, and finishes the chain with a clasp. Click here to check out the DVD.
What knitters call I-cord is also called spool knitting, or using a knitting nancy or knitting spool. This ultimately gives a similar effect as Viking Knit, but uses a different method. Instead of feeding the wire through existing loops like in Viking Knit, the wire is wound around (typically) 4 pegs. These pegs hold loops of wire, and as more wire is wound around the pegs, you lift up the loop, slide it over the new wire, and off the peg, joining the chain. This makes a real knit stitch, as if you'd made it on double-pointed needles (but much less fiddly). This chain doesn't usually have the even, sleek stitches that Viking Knit has, and can take longer because of having to pass the loops over.
Both styles of chain can easily be embellished by sliding beads onto the wire. With spool knitting, be sure to slide them on ahead of time, because the end of your wire disappears into the chain!
You can make your own spool with a piece of wood with a hole drilled in it and cotter pins, or I'm sure you can find a knitting nancy tool at different stores.