Latex: customising syntax highlighting in vim

I made some custom commands to make writing equation blocks easier:

% Equation starters
\newcommand{\eqn}[1]{$$#1$$}
\newcommand{\eqns}[1]{\begin{equation*}$$#1$$\end{equation*}}
\newcommand{\eqna}[1]{\begin{align}$$#1$$\end{align}}
\newcommand{\eqnas}[1]{\begin{align*}$$#1$$\end{align*}}


This is so that

$$e^{i \tau} = 1$$


gets simplified to

\eqn{
e^{i \tau} = 1
}


In the second line, the syntax highlighting inside the \eqn{} block is wrong, because vim doesn't realise it's maths. Now, I just need to tell vim what I have done. I have no idea how, though.

" Add custom equation delimiters to syntax highlighting.