- yuppie is short for Young Urban Professional
- DINKs is short for Dual Income, No Kids
It sounds to me like they are not real acronyms, not being strictly one letter per word.
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
Neither of these are portmanteaux. To form a new word, a portmanteaux has to use parts of at least two words, more than their initial letters but less than the complete words (although it can use one of the words completely, but not all of them). If you can imagine a scale (with "0' on the left and "10" on the right), portmanteaux are in the middle range, abbreviations are on the far left and compound words on the far right.
With your examples, you would have to have something like "youbanfessional" or "duacomenoids" for them to be portmanteaux. Both "yuppie" and "DINK" are examples of initialisms. DINK is an initialism that is a strict acronym- it is made from the first letters of each word and is pronounced as a new word.
Because most people don't know of any other names with which to describe it, "yuppie" is sometimes referred to as an acronym or an abbreviation. It is neither since: 1) it is not strictly formed from the first letters of it's forming words although it is pronounced like an acronym, and 2) it is not an abbreviated form of a full word. It isn't a portmanteau because we know its forming words and it is not formed from parts of them. The word "yuppie" is a simple initialism.
I hope this helps.
In the UK, there are pedestrian crossings called 'Pelican' crossings. They get their name because they are PEdestrian LIght-CONtrolled crossings. Similarly, 'Puffin' crossings are Pedestrian User-Friendly INtelligents crossings. Finally, another crossing, designed for both pedestrians and cyclists, is named the 'Toucan' crossing because the TWO CAN cross here. All are examples of contrived portmanteau words with three, four and two elements respectively.
DINKs does seem to be an acronym to me, because the s at the end is only intended to express plurality. Yuppie is a bit different of a situation, because I don't think that it really fits either description well. It wouldn't be an acronym, for reasons you have already listed, and portmanteaus are usually more fluid than that. I would classify that as slang, because it's an abbreviated form of the phrase that doesn't really have a consistent form.
Generally if something doesn't fit the description of an acronym or portmanteau you can just call it an abbreviation. In the case of other vernacular things like Yuppie though, slang is usually best fitting.