I think the post gets the basic definitions, although its use of "epistemological" requires its own extensive definition, and so doesn't really help with a simple explanation.
Think of it this way: A "concept" is the fundamental building block of human thought. It represents an abstraction from "reality". External/Physical objects always relate to our conceptual interpretation of real-world objects, and vice-versa.
external reality ⟺ internal concept
For example you have an internal concept of "apple", based on experience. It has a certain shape, color, smell, texture, flavor, food value, etc. When you perceive an apple in your environment, through one or another of your senses, you recognize it as an "apple" by its similarity to your concept of "apple".
Concepts don't have to be related to physical objects. They can be abstractions like "love" or "health" or "independence". All of these are still related in some way to real-world experiences.
"Notion" is a subset of "concept". Notions are generally vague, meaning they are incomplete or incompletely defined. For example, a teacher might introduce the "notion" of epistemology to her students, meaning that she may explain the general definition, and some of the basic ideas, but not go into full detail about its philosophy, practice, or limitations.
So a vector -- or rather, the subject of vector-based mathematics, is a "concept" with a fairly clear definition. In the same way as the example above, a teacher might give students a "notion" about what vector mathematics is, by showing a couple of examples, but not going into any great detail.
"Notion" is also used for things like guesses, including more formal guesses like conjectures or hypotheses. For example:
Modern cosmologists have come up with a notion about what they call "dark matter" and "dark energy" to explain certain observed phenomena in the universe, but they are still unable to explain what these actually are.
Because a notion is a kind of concept, it would also be fine to say
Cosmologists have come up with the concept of dark matter/energy to explain certain observed phenomena in the universe, but they are unable to explain what these actually are.