Rotation amplitudes, Vmax are usually measured in one of two ways. Long slit optical spectra aligned with the galaxy major axis trace out the rotation curve, allowing the amplitude to be measured.
Single dish observations of the neutral hydrogen 21cm line profile. Measuring the full width at 20% or 50% of the height corresponds closely to (twice) the rotation amplitude. One is effectively integrating over the spider diagram. For a standard flat rotation curve, a steep sided profile is formed, which gives a well defined width.
More examples of 21cm profiles are here: o-link