It's already late for holiday reservations (prices are up). One of the cheapest days to travel outbound would be 12/10, cheapest flight on China Eastern for $1059. JFK-SGN-JFK. China Eastern is an inferior carrier in my opinion.
For open jaw it's considerably more - both ANA and Asiana at $1626.
Asiana JFK-SGN/HAN-JFK and ANA IAD-SGN/HAN-JFK - both excellent carriers. Note that ANA (as well as Korean) fly to both IAD and JFK. Korean is priced a little higher, but Asiana is rated higher.
I personally booked an open jaw into Bangkok and return from Hanoi in Nov-Dec. I booked back in Jan for $1241. on Asiana (today, the same flights would cost me over $4000.!)
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(regarding flight times, you will see that many flights returning to the States have a lengthy transit time. Asiana and Korean will give you a free hotel room for that transit in Seoul if you request it after booking - just call their toll free phones in the USA)