Same notation/terminology for union of sets and concatenation (Kleene star)?
Wikipedia is using $\cup$ in its usual meaning, set union. When Wikipedia defines
$$
V^* = \bigcup_{n \geq 0} V^n = V^0 \cup V^1  \cup V^2 \cup \cdots,
$$
it literally means that $V^*$ is the union of the sets $V^0,V^1,V^2,\ldots$

The concatenation happens in the definition of $V^n$, which can be defined as follows: $V^0 = \{\epsilon\}$, and $V^{n+1} = \{ xy : x \in V^n, y \in V \}$. In words, $V^n$ is the concatenation of $n$ words from $V$.