In an original Duo Dock with System Software 7.1.1, the additional VRAM offers additional colors, but not additional resolutions. In order to access additional resolutions on the Multiple Scan 17 Display, you either need a display adapter (because the Dock does not recognize the sense pins on the monitor cable), or you need a newer version of System Software.
Display adapters do not increase the number of resolutions a Duo Dock can handle, but they do give you access to all the resolutions that the Duo Dock can support. The highest resolution supported on the original Duo Dock is the 16-in. monitor resolution of 832 x 624.