If you have not figured out how to add your unit already there is a tutorial for adding units to FA here. Use the table of contents to go to the actual adding the units section.http://supcom.wikia.com/wiki/Beginners_ ... _Structure
Should be the the same I believe for vanilla or very close. There are quite a few files that need that your unit needs to be added into. I am not at home right now and do not have vanilla loaded on my lap top. Be a few days before I get home but the tutorial even though FA should give you 99% of what you need to do. Could very well be the all the files. I never tried adding a unit to vanilla before.
You can also down load an old vanilla mod if there are any left and look at the files that the modder used. I think the most difficult part in vanilla if I remember correctly is adding the icon place holder. There used to be a tutorial around here for that and may still be here somewhere. I think Goom was the author of that tutorial. If you have trouble with that and can not find the tutorial I may have it stashed somewhere but probably at home.