Sure, you can use that old wire if it's in fairly good condition. I would test the continuity of it from end to end first, and wiggle it around a little to be sure the entire length is good before you count on it as your halloween wire. I recommend 14-2 or thicker (12-2 or 10-2 has heavier copper wire, and is more expensive and sort of overkill). I have used 16-2 as well, but that's about the practical limit as far as how small you can go using the vampire clips.Can you reuse old landscape lighting wire? I have 200 feet of used wire that has had lights clipped to it?
We relocate lights for customers all the time as thier trees grow, or as thier lighting requirements change. The punctures that are created by the vampire clips do cause "Some" degredation of the copper inside, but in general, this type of wire can last a VERY long time, especially if you only use it for 1 month a year and then store it in a dry location. It's not uncommon for this type of wire to last 10-20 years, even when its in constant exposure to the elements. Often it is buried in damp mulch beds for it's entire life and does just fine.