Not all CRT Studio Displays are VGA -- some are ADC. The earlier models, like the 17" and 21" Apple Studio Displays that have a graphite housing around them and Apple logos on the sides and back are VGA. The 17" Apple Studio Display that's got the flat CRT screen and transparent housing is ADC.
If it's got graphite housing and its 21", then its got a VGA connector. Those are excellent monitors, not only because of the Trinitron screen, but they have aperture grilles instead of fixed dot pitches, which means, basically, that the image on the screen will be a lot sharper than with a conventional CRT without an aperture grille. One thing you may notice about that monitor is that it's got two very fine lines that run horizontally across the screen about 1/4 up from the bottom and 1/4 down from the top. They're barely noticeable in daily work, but you can find them if you look -- that's the aperture grille, and it's completely normal to see those lines.