Why is it experimentally difficult to accurately find the critical volume of a real gas?

I was reading up on the derivation of the values of various critical state functions( $\ce{Vc}$, $\ce{pc}$ and $\ce{Tc}$) from the Van der Waals equation (Physical Chemistry by G.W. Castellan). There, he was about to express a and b in terms of these critical state functions, when I came across this line:



> 
> Since experimentally it is hard to determine $\ce{Vc}$ accurately, it would be better if a and b
>  could be obtained from $\ce{pc}$ and $\ce{Tc}$ only.
> 
> 
> 


What is exactly the difficulty in finding $\ce{Vc}$ accurately?
I found [this](https://www.oreilly.com/library/view/heat-and-thermodynamics/9789332511231/xhtml/ch5sec6.xhtml) source stating that:



> 
> Of the three constants, measurement of critical temperature and critical pressure is very easy. The measurement of critical volume is very difficult because it varies considerably for slightest variation of temperature and pressure near the critical region; hence for the measurement of critical volume, the substance must be kept in perfectly critical temperature and pressure.
> 
> 
> 


**My two cents**


If $\ce{Vc}$ is so short-lived and unstable, then any variation in the same would almost instantly affect the vapour pressure of the gas phase above it, and thereby $\ce{pc}$ would also not be a very reliably measured quantity. Then maybe we would end up with two variables(a and b) and one equation($\ce{Tc}$).


Can somebody explain the procedure of measurement of critical state function here, and why only $\ce{Vc}$ causes problems,and the other two remain appreciably accurate?


