I'm certainly no expert on this but I found some interesting things out concerning Knox's question.
The U.S. has had an embargo in place against Cuba since 1962 so I don't know in who's lap you want to place responsibility. I think the embargo is and was justified considering that in the process of their "revolution", Castro and his supporters nationalized (read: stole) extensive assets belonging to U.S. citizens and corporations. It doesn't take much of an intellectual stretch to see why it might be undesirable to do business with a regime that would be profiting from the investment of labor and capital of our citizens. Add that to Cuba's alliance with the Soviet Union and it is clear why we should have set up an embargo.
As for the period following the fall of the Soviets, it has been the policy of the U.S. to encourage the spread of democracy, something Cuba hardly qualifies as, and the Helms-Burton Act of 1996 strengthened the embargo to bring additional pressure on the Cuban Government to become more democratic:
The Cuban Liberty and Democratic Solidarity (Libertad) Act of 1996 (Helms-Burton Act, Pub.L. 104-114, 110 Stat. 785, 22 U.S.C. § 6021–6091) is a United States federal law which strengthens and continues the United States embargo against Cuba. The act extended the territorial application of the initial embargo to apply to foreign companies trading with Cuba, and penalized foreign companies allegedly "trafficking" in property formerly owned by U.S. citizens but expropriated by Cuba after the Cuban revolution. The act also covers property formerly owned by Cubans who have since become U.S. citizens.[1]
The law was passed on March 12, 1996 by the 104th United States Congress. The bill, which had been tabled in late 1995 after Senator Helms was unable to overcome several Democratic filibusters, was reintroduced prompted by an episode that happened a month earlier. On February 24, 1996, Cuban fighter jets shot down two private planes operated by a Miami based anti-Castro Cuban refugee support group called Brothers to the Rescue (Hermanos al Rescate). [2] [3].
http://en.wikipedia.org/wiki/Helms-Burton_Act
It is fair to say that the average Cuban hasn't had a great time during the embargo (unless speaking with Sean Penn, Harry Belafonte, Michael Moore, etc.) but that might have just as much to do with the economic inefficiencies inherent in a planned economy. Cuba certainly has plenty of other trade partners. As for the passions of South Florida's Cubans, I think they are justified in their feelings. They are the ones after all who were forced to flee the "revolution".
As for China, we began to normalize trade relations with that nation in the early 1970's. While the Maoist regime of China is Communist, the U.S. and China shared a common adversary in the Soviet Union. Clearly, our economic partnership with China is a source of great concern in the present day and continues despite seriously questionable practices by the Chinese government, however to impose an embargo in this case would be foolish beyond belief if not impossible altogether.
Some key numbers to also keep in mind:
China GDP = $10.17 Trillion
Cuba GDP = $45.51 Billion
Vietnam GDP = $262.8 Billion
I think the initial question is really one of apples and oranges but in a way that is probably appropriate as the consistent application of foreign policy has not been a strong point in the history of the U.S. government.