% test.tex
\documentclass{book}

\usepackage{fixfoot}
\renewcommand{\thefootnote}{\alph{footnote}}

\makeatletter
\renewcommand\@fixed@footnote[3]{%
  \stepcounter{@#1@fftn@serial}%
  \ifnum\csname @#1@fftn@pagelim\endcsname<\csname c@@#1@fftn@serial\endcsname
    \expandafter\let\expandafter\@tempa
      \csname @#1@fftn-\the\csname c@@#1@fftn@serial\endcsname\endcsname
    \ifx\@tempa\relax\else
      \global\expandafter\let\csname @#1@fftn@pagelim\endcsname\@tempa
    \fi
    \footnote{#3}%
    \expandafter\xdef\csname @#1@fftn@footnote\endcsname{\thefootnote}%
  \else
    \protected@xdef\@thefnmark{\csname @#1@fftn@footnote\endcsname}%
    \@footnotemark
  \fi
  \protected@writeaux\relax{\protect\@process@fftn@mark{#1}%
    {\the\csname c@@#1@fftn@serial\endcsname}{\noexpand\thepage}}%
  #2%
}
\makeatother

\newcommand{\VS}[1]{$^{#1}$ \label{#1} }

\DeclareFixedFootnote{\sic}{\emph{sic}}

\begin{document}
\VS{1} First.\footnote{Letter a.}
\VS{2} Second SIC.\sic{}
\VS{3} Third.\footnote{Letter c.}
\VS{4} Fourth SIC.\sic{}
\end{document}