Ever since upgrading to High Sierra, I am no longer to connect to my Epson projector

Connecting via a USB cable is not supported.

It used to work with some dongles or docks via USB using "displaylink" third party drivers. They used some tricks seemingly involving nonpublic system API.

After the changes on the graphics architecture around 10.13.4 (needed, it seems, to support eGPUs) these drivers no longer worked. There appear to be workarounds and it is possible that a solution may come forward in Mojave.

In the meantime, is there any reason not to connect in a standard supported way - via a miniDisplayPort or HDMI connection?

Hi,

"Ever since upgrading to High Sierra"

Safe Mode boot deletes system caches that can help after a macOS update or upgrade.

To start up in safe mode:

1. Start or restart your Mac, then immediately press and hold the Shift key. The Apple logo appears on your display. If you don't see the Apple logo, learn what to do.
2. Release the Shift key when you see the login window. If your startup disk is encrypted with FileVault.

To leave safe mode, restart your Mac without pressing any keys during startup then try your projector.

Use safe mode to isolate issues with your Mac - Apple Support

tanyaiback wrote:

I tried a HDMI to vga connector, but when I plug the HDMI into the MacBook, the screen goes black

I am afraid that won't work, either. The HDMI port on the MBP does not carry a VGA signal.

A miniDisplayPort to VGA would work.