You do get zero density for $x^k - y^k$ with, say,  $x,y > 0,$ as
$ x^k - y^k = (x-y) (x^{k-1} + \cdots + y^{k-1} ) $ and either $x=y$ or $| x - y| \geq 1,$ so the number of $(x,y)$ pairs with $0 < x^k - y^k \leq n$ is no larger than $n^{2/(k-1)}.$ As soon as $k \geq 4$ we get zero density. 

However, it is now suspected that $\pm x^3 \pm y^3 \pm z^3$ gives full density, that being 7/9. Also, by easy identities, all numbers are the mixed sum of five cubes, so $v(3) \leq 5$ 

I would start by asking your question with three variables instead of five...