If Vietnam and the USA have normal diplomatic and business relations, why doesn't Cuba-USA relationship improve? Why doesn't the USA normalize its diplomatic relations with Cuba?
Is it a duplicate? Not exactly, but close and its answers aren't so hot either, so here's a cynical take on things (this kinda question tends to annoy Americans as, IMHO, their reasons for the embargo are fuzzy at best).
Foremost: not antagonizing the Cuban expat community in Florida which is often a battleground state, election-wise. Anyone remember Elian Gonzalez? Might have very well tilted the Florida vote that elected Bush.
Communism/Dictatorship. Yup, Cuban is Communist (note: I dislike Communism myself, but stating this is no so much part of the answer as it is to make clear I am not coming into this from the POV of being pro-Cuba or pro-Communist. I am just critical of the USA's specific behavior towards Cuba). But the US can hold its nose in other cases. True, it is unfree but so are Saudi Arabia, Thailand and a whole lot of other countries that the US is good buddies with. It's the easy claim for US domestic consumption however.
The upsides to normalizing to the US aren't huge. Cuba doesn't have that much to trade/offer and their enmity comes at a very low cost to the US.
Vietnam is slightly different - oddly enough, I think a lot of reconciliation happened through the recovery efforts to find the remains of MIA US soldiers. Vietnam doesn't like China much so can be a useful regional ally. Finally, there's the often quoted fig leaf of "we didn't really lose the war, we coulda won, we withdrew", letting the US claim dignity (see next point).
Last but not least: the US hates to lose face They did w Cuba and Iran (might apply in the future to Afghanistan too). No US politician wants to open to themselves to domestic political attacks by appearing soft on Cuba or Iran so a slightly over-the-top policy that has limited downsides for the US is going to stick around for a long time unless Cuba "admits its wrongs".
P.S. After spending 6 decades complaining about the US Great Satan, why does Cuba want to trade with it at all? Yes, yes, I know about the dubious application of US extraterritoriality principles, but when you pick an argument with a big guy, don't be too surprised if you get punched in the nose.
FWIW, I think Obama did the right thing, not an aberration, but there's clearly not a big popular push in the US to kiss and make up.
Also the Monroe Doctrine, to some extent, plays a role. Like Nicaragua and Venezuela, Cuba is in the same hemisphere as the USA, but Vietnam not. It is unacceptable to have an enemy living next door while the enemy living in a neighboring city can be tolerated. There is also an historical dimension. The situation between Cuba and the USA has some similarities with situation between Ukraine and Russia. Cuba was a US colony for some time and, even after becoming an independent state, it was strongly influenced by the US. If staying in that paradigm, Batista was a US version of Yanukovych.