I'm not an experienced calibrator, but I do my own calibrations and I have the basic knowledge of calibrating.
The white balance allows you to adjust red, green and blue to set them so that they're tracking where they should be to create a neutral grayscale. The ST/GT only allow you to adjust 2 points (high and low) while the VT50 allows you to adjust 10 points (10-100 in 10 steps). This allows you to get RGB tracking ruler flat from 0-100 IRE whereas the 2pt system generally won't be as flat/perfect because there's fewer adjustment points. With a 10pt system, you can also use it to raise/lower gamma.
If you're a little confused, google image search RGB tracking or look at the calibration reports from Chad B of the VT50 and ST50. Or maybe a more experienced calibrator can give a more technical yet easier to understand reply or correct any mistakes I've made.
A color management system (CMS) allows you to adjust the primary (red, green, blue) and secondary (yellow, cyan, magenta) color points to get them as close as possible. I'm not sure how good the VT50's CMS is, but it should definitely allow you to get the color points closer than not having one at all. But the ST/GT have fairly good color despite not having a CMS.
So with that said, while the VT50 is more accurate after calibration, because the ST/GT are already so good, what this all translates to on screen isn't as significant as it may sound. Colors will just look a bit more as they 'should'.