Suppose that the tangent at $V=[\vec v]$ on C passes through $U=[\vec u]$.
Then $\vec v^{\top}M\vec u=0$, and, transposing, $\vec u^{\top}M\vec v=0$.
Also, as V is on C, $\vec v^{\top}M\vec v=0$.
Any point T on the line UV may be written as $T=[\vec t]$, where $\vec t=a\vec u+b\vec v$, for some $a, b$.
Expanding each side, and using the equalities above,
we see that $\vec t$ satisfies the equation $\vec u^{\top}M\vec u\,\vec x^{\top}M\vec x =\vec u^{\top}M\vec x\,\vec u^{\top}M\vec x$.
Similarly, if the tangent at W passes through U, then each point of UW satisfies the equation.
Finally, the given equation is quadratic in $\vec x$, so can represent no more than two lines.
Of course, by La Hire's Theorem, the chord VW in the above theorem is the polar of U.