Step 1: Go to CDNN and order a genu-ine USGI Upper. (might be a little late for most of VietNam, they're mid-70's vintage I think)
Step 2: Go to Moore Militaria or Sherluk and order a genu-ine USGI buttstock and grip. The one I got still has some tape and paper on the butt from where an ID number was taped once upon a time.
Step 3: Order a stripped lower receiver and the miscelleneous pieces parts. You may need a buffer assembly, you'll definately need a bolt carrier and semi-auto trigger group, etc.
If you want to get particularly authentic, it'll take some looking and patience to find the various take down pins, selector, etc... ar15.com has a retro forum that's been a help.
I think my A1 clone is in the $650-700 range, going as above. Cheaper than most SP1s, and it has the nice cache of being (in most parts) actual gov't issue.