Our "getting back on track" with Vietnam wasn't just a consequence of our attitude, it was to an equal degree a consequence of theirs. Our relations with former antagonists, like so many other things in this world, aren't defined solely by us.
Former antagonists that are now friends (some became friends immediately after the conflict)

England
Mexico
Germany
Japan
Vietnam
Iraq (friend, foe, friend)
Nicaragua

Former friend and now antagonist:
Iran
Venezuela
Cuba

Iffy status:
China
Russia
Pakistan (can also fit under the currently hostile category)

Currently Hostile:
North Korea
Iran
Non-state groups

Long term friends (not fair weather friends):
UK
Canada
Australia

Wouldn't take too much to bring Cuba into the fold of friend again if we could limit the influence of the far right Cubans in the U.S. who are doing more harm than good for their fellow Cubans.