Is the category 2-Vect monoidal closed?

Conceptually, $2Vect$ should be viewed as a sub-2-category of the 2-category $Bim$ of $k$-algebras with bimodules between them. In fact, when $k$ is algebraically closed, $2Vect = Bim^{fin}$ is exactly the 2-category of finite $k$-algebras and bimodules between them, because every finite $k$-algebra is Morita equivalent to $\oplus$-sum of copies of $k$. And the inclusion $2Vect \to Bim$ sends $n \mapsto k^{\oplus n}$, landing in commutative algebras.

Note that $\boxtimes$ corresponds to $\otimes$ of algebras, which is the coproduct for the sub-2-category $CAlg$ of commutative algebras and homormophisms, so it is definitely not closed when restricted to $CAlg$ (a cocartesian monoidal structure is closed only in trivial cases). But it's a general fact that for any algebra $A$, the opposite algebra $A^\circ$ is dual to $A$. Having duals in particular implies that the monoidal structure is closed.

As Tim says, for a conceptual ("coordinate-independent") picture we should start with $\text{Bim}(k)$, the 2-category of $k$-algebras, $k$-bimodules between them, and $k$-bimodule homomorphisms, which we can think of as a reasonably well-behaved 2-category of $\text{Mod}(k)$-modules. By Eilenberg-Watts we can equivalently think of this 2-category as the 2-category whose objects are categories of the form $\text{Mod}(A)$, $A$ a $k$-algebra, and whose morphisms are $k$-linear cocontinuous functors.

This 2-category is closed monoidal with monoidal structure given by tensor product, which has a universal property: cocontinuous $k$-linear functors out of $\text{Mod}(A \otimes_k B)$ correspond to $k$-linear functors out of $\text{Mod}(A) \times \text{Mod}(B)$ which are cocontinuous in each variable. Every object is dualizable, with dual given by $A \mapsto A^{op}$, and hence the closed structure is given by $[A, B] = A^{op} \otimes_k B$. This reflects the fact that $\text{Mod}(A^{op} \otimes_k B)$ is precisely $(A, B)$-bimodules over $k$, which are precisely the morphisms $A \to B$ in this 2-category.

KV considers the sub-2-category of this consisting of $k$-algebras of the form $k^n$ and finitely generated projective $(k^n, k^m)$-bimodules over $k$, which are described explicitly by $n \times m$ matrices of finite-dimensional $k$-vector spaces. These morphisms are singled out by the property of also being dualizable (that is, they have adjoints; see this blog post), and these $k$-algebras are 2-dualizable in addition to being dualizable, but this does not exhaust all 2-dualizable objects. In general the 2-dualizable objects are the $k$-algebras $A$ which are finite-dimensional and separable over $k$ (and this definition, unlike the KV definition, should satisfy Galois descent); the algebras $k^n$ exhaust these iff $k$ is separably closed. These can be used to define 2-dimensional topological field theories via the cobordism hypothesis. For example, if we take $A$ to be a twisted group algebra for a finite group $G$ with twist defined by a $2$-cocycle $\alpha \in H^2(G, k^{\times})$, then the resulting TFT is 2d Dijkgraaf-Witten theory with gauge group $G$ twisted by $\alpha$.