I received my TB15 dock yesterday and connected it to my XPS15 9550 allowing me to finally connect to an external monitor, The monitor is a several year old Dell with 1920/1200 resolution and I connected it to the TB15 via a standard VGA cable. I immediately noticed that any text displayed wasn't nearly as crisp as when I use the Monitor directly from an older XPS laptop which has VGA output. I called Dell Support but other than checking for current version of drivers could offer no suggestions.
I won't be able to use the monitor connected this way as it is far to blurry so will return the TB15. Is this blurriness to be expected or does anyone have a workaround? I see comments on some lower priced adapters that the video quality is somewhat degraded when using the usb-c connector but did not expect this from the TB15.
You might try a HDMI to VGA adapter and connect that to the 9550'S onboard HDMI out plug. I don't know about the quality of those adapters, but it might be worth a try. There are a number of issues with the TB15 that really need to be resolved before it becomes viable.
You didn't say what monitor model you have, but if it's 1920x1200 it has some digital inputs, HDMI or DVI, if not even DisplayPort. If it uses DVI get yourself a simple HDMI to DVI cable and connect this to the dock. Don't use VGA cable for LCD monitors, if you absolutely have to - there are usually items on the monitor menu to tune analog signal, they are somewhere in manual image adjustment menu. One of the items will be "auto adjust" - enabling it will get you best possible picture, but still nowhere near the sharpness of digital signal.
My monitor is a DELL U2412M, it has VGA, DVI and DisplayPort inputs, no HDMI. I'll try and locate a HDMI to DVI cable. I tried the auto adjust with the TB15 and it made no difference. I see there are USB-C to VGA adapters available but I'm not sure if the same issue with clarity will happen when using this (is the clarity problem specifically with the USB-C interface)