Yup. Nobody made, as far as I know, an HDI45 to VGA adapter. If it has that connector, it is a 6100, 7100, or 8100.
Another option, my personally preferred route on the x100 Power Macs, is to get an Apple AudioVision display, which has that connector.
The connector collects video for the monitor, ADB, sound, and allegedly for the A/V models, an analog video input signal, all into one cable that goes from the system to the monitor.
Apple's idea at the time was to make everything a cleaner setup (which, it certainly would be if you had an 8100) and build it out into a bit more of an ecosystem.
Other options include getting the adapter and using a VGA adapter on it, or getting a NuBus or HPV video card to use with the system.