Vinogradov systems with a slice off

Let $I_{s,k,r}(X)$ denote the number of integral solutions of the modified Vinogradov system of equations $$x_1^j+\dots +x_s^j=y_1^j+\dots +y_s^j(1\le j\le k\text{, }j\mathrm{\ne }r),$$ with $1\le x_i,y_i\le X$ $(1\le i\le s)$. By exploiting sharp estimates for an auxiliary mean value, we obtain bounds for $I_{s,k,r}(X)$ for $1\le r\le k-1$. In particular, when $s,k\in \mathbb{N}$ satisfy $k\ge 3$ and $1\le s\le (k^2-1)\mathrm{/}2$, we establish the essentially diagonal behaviour $I_{s,k,1}(X)\ll X^{s+ϵ}$.