Specify prefix within the source in a citation

Question

I have a problem. I would like to have an indirect quote with a prefix flag. Is it possible to design a new \cite command that outputs the source as follows [vgl. 1]. As you can see, the vgl. is added. This is what I always want when I write \citewp for example. Is there such a possibility?

What I have

Let's cite! The Einstein's journal paper [1] (~\cite) are physics related items. 

What I want

Let's cite! The Einstein's journal paper [vgl. 1] (~\citewp) are physics related items.

\documentclass{article}
\usepackage[utf8]{inputenc}
\RequirePackage{filecontents}
\begin{filecontents*}{\bibliography.bib}
@article{einstein,
  author  = {Albert Einstein},
  title   = {{Zur Elektrodynamik bewegter Körper}. ({German}) 
             [{On} the electrodynamics of moving bodies]},
  journal = {Annalen der Physik},
  volume  = {322},
  number  = {10},
  pages   = {891--921},
  year    = {1905},
  DOI     = {http://dx.doi.org/10.1002/andp.19053221004},
}
@online{ApoG,
  Address = {03.08.2013},
  Edition = {15.10.1980},
  Publisher = {juris GmbH},
  Title = {ApoG - Gesetz {\"u}ber das Apothekenwesen},
  Url = {www.juris.de/purl/gesetze/_ges/ApoG},
  Year = {1980},
}
\end{filecontents*}
\usepackage[ngerman]{babel}
\usepackage[%
backend=bibtex      % biber or bibtex
%,style=authoryear    % Alphabeticalsch
,style=numeric-comp  % numerical-compressed
,sorting=none        % no sorting
,sortcites=true      % some other example options ...
,block=none
,indexing=false
,citereset=none
,isbn=true
,url=true
,doi=true            % prints doi
,natbib=true         % if you need natbib functions
]{biblatex}
\DefineBibliographyStrings{ngerman}{%
    urlseen = {aufgerufen am}
}
\addbibresource{bibliography.bib}  % better than \bibliography
\begin{document}
Let's cite! The Einstein's journal paper ~\cite{einstein} are physics related items.
\printbibliography[title=Literaturverzeichnis]
\end{document}

Answer 1

biblatex's cite commands have up to two optional arguments

\autocite{<keys>}
\autocite[<post>]{<keys>}
\autocite[<pre>][<post>]{<keys>}

If only one optional argument is given, it is interpreted as the <postnote> argument. If two optional arguments are given, the first is the <prenote>, the second the <postnote>.

You can give a <prenote> without a <postnote> if you leave the <postnote> empty

\autocite[<pre>][]{<keys>}

• The <postnote> is normally displayed after the citation label and traditionally contains page ranges.
• The <prenote> is displayed before the citation label and often contains specifiers like "cf."/"vgl.".

So can just go with

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage[
  backend=bibtex,
  style=numeric-comp,
  sorting=none,
]{biblatex}
\addbibresource{biblatex-examples.bib}
\begin{document}
Lorem \autocite[vgl.][]{sigfridsson}
Ipsum \autocite[vgl.][380]{sigfridsson}
\printbibliography[title=Literaturverzeichnis]
\end{document}

I would just stick to using the <prenote> argument, but if you insist on defining a new command, you can try the following, which will "inject" a <prenote> equal to "vgl." into your \cite.

You can then use \autocitewp without argument and will get "vgl.". You can still both a <prenote> argument (which will just override the "vgl.") and a normal <postnote>.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage[
  backend=bibtex,
  style=numeric-comp,
  sorting=none,
]{biblatex}
\makeatletter
\newrobustcmd*{\autocitewp}{%
  \AtNextCite{\def\abx@field@prenote{vgl.}}%
  \autocite}
\makeatother
\addbibresource{biblatex-examples.bib}
\begin{document}
Lorem \autocitewp{sigfridsson}
ipsum \autocitewp[380]{sigfridsson}
dolor \autocitewp[cf.][381]{sigfridsson}
\printbibliography[title=Literaturverzeichnis]
\end{document}

Answer 2

You can define a new command like this: \newcommand{\citewp}[1]{\cite[vgl.][]{#1}}:

\documentclass{article}
\RequirePackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@article{einstein,
  author  = {Albert Einstein},
  title   = {{Zur Elektrodynamik bewegter Körper}. ({German}) 
             [{On} the electrodynamics of moving bodies]},
  journal = {Annalen der Physik},
  volume  = {322},
  number  = {10},
  pages   = {891--921},
  year    = {1905},
  DOI     = {http://dx.doi.org/10.1002/andp.19053221004},
}
@online{ApoG,
  Address = {03.08.2013},
  Edition = {15.10.1980},
  Publisher = {juris GmbH},
  Title = {ApoG - Gesetz {\"u}ber das Apothekenwesen},
  Url = {www.juris.de/purl/gesetze/_ges/ApoG},
  Year = {1980},
}
\end{filecontents*}
\usepackage[ngerman]{babel}
\usepackage[%
backend=bibtex      % biber or bibtex
%,style=authoryear    % Alphabeticalsch
,style=numeric-comp  % numerical-compressed
,sorting=none        % no sorting
,sortcites=true      % some other example options ...
,block=none
,indexing=false
,citereset=none
,isbn=true
,url=true
,doi=true            % prints doi
,natbib=true         % if you need natbib functions
]{biblatex}
\DefineBibliographyStrings{ngerman}{%
    urlseen = {aufgerufen am}
}
\addbibresource{\jobname.bib}  % better than \bibliography
\newcommand{\citewp}[1]{\cite[vgl.][]{#1}}
\begin{document}
Let's cite! The Einstein's journal paper ~\citewp{einstein} are physics related items.
\printbibliography[title=Literaturverzeichnis]
\end{document}