Flushright and whitespace

\flushright etc. introduce additional vertical white space, making \vspace 'necessary', I recommend \raggedright etc. for this.

\item does not have a mandatory argument!

In addition, I recommend the enumitem package for fine control of the vertical spacings in an itemize list.

\documentclass{article}

\usepackage{enumitem}

\usepackage{showframe}% Remove this later on

\newcommand{\resitemwithdate}[2]{%
\item 
  \begin{minipage}[t]{.80\linewidth}
    \raggedright #1%
  \end{minipage}
  \hfill
  \begin{minipage}[t]{.15\linewidth}
    \raggedleft #2%
  \end{minipage}
}

\begin{document}
\begin{itemize}[itemsep=0pt]
    \resitemwithdate{A}{B}
    \resitemwithdate{A}{B}
    \resitemwithdate{A}{B}
\end{itemize}
\end{document}

And now for something completely different!

I use the listliketab package, to emulate an itemize environment, customised with enumitem, and the date-time2 package to format the date, within a tabularx environment:

\documentclass[british]{article}%
 \usepackage{enumitem, tabularx, listliketab}
 \usepackage{babel} %
 \usepackage[british]{datetime2, }%
 \usepackage{datetime2-calc, fmtcount}%
 \DTMlangsetup[en-GB]{abbr, ord=raise}
  \usepackage{lipsum}%
 \usepackage{showframe}% Remove this later on
\newlist{myitemize}{itemize}{1}
\setlist[myitemize]{wide=0pt, label=\textbullet, leftmargin=*}
\newcommand{\resitemwithdate}[2]{%
\item
  \begin{minipage}[t]{.80\linewidth}
    \raggedright #1%
  \end{minipage}
  \hfill
  \begin{minipage}[t]{.15\linewidth}
    \raggedleft #2%
  \end{minipage}
}
\let\tabitem=\textbullet

\begin{document}

\storestyleof{myitemize}
\begin{listliketab}
  \begin{tabularx}{\linewidth}{@{}L >{\hsize=1.6\hsize}X >{\hsize=0.42\hsize\raggedleft\arraybackslash}X@{}}
    \tabitem & \lipsum[11] & \Today \\
 \tabitem & \lipsum[3] & \Today%
    \end{tabularx}
\end{listliketab}

\end{document} 

As exercise:

\documentclass{article}
\newcommand{\resitemwithdate}[2]
{
    \setlength\itemsep{0pt}%
    \item   \begin{minipage}[t]{.80\linewidth}\raggedright
                #1
            \end{minipage}
            \hfill
            \begin{minipage}[t]{.15\linewidth}\raggedleft
                #2 
            \end{minipage}% 
}

\usepackage{showframe}                     % remove in real document
\renewcommand*\ShowFrameColor{\color{red}} % remove in real document

\begin{document}
\begin{itemize}
    \resitemwithdate{A}{B}
    \resitemwithdate{A}{B}
    \resitemwithdate{A}{B}\break
    \resitemwithdate{A}{B}
\end{itemize}
\end{document}