I dont understand which you are looking for Vintage Fluted or current Zero Setback. They are mutually exclusive.
I noticed this Campagnolo post on eBay.
It looks like a 1980s C-Record era post, not bad looking
and probably good for the period of the bike.
For a Zero Setback post you'll need a VERY modern (current) seatpost...
and Thompson's "Elite" posts are the only ones I know of EVER
made in that diameter...
so, that style is maybe not appropriate if you're trying to keep faithful to the period?