I see that you're trying the same thing I did. Some advice, DON'T DO IT!
Reason I'm explaining this is I have the same monoprice adapter and while it works good, the board itself has an issue with high contrast, which causes a lot of video to be hit with "white crush". If you have another source that goes before this, you can turn the contrast levels down and it shouldn't be an issue. My alternative solution when I used mine was to run it through a cga/vga board as a pass-thru and it really cleaned the signal up, but as a warning, I ended up with some kind of pulsing effect that runs as signal interference.
Fudah suggested that due to how Dreamcast outputs it's signal, most of the scaling done on HDTV sets (VGA to HDMI scalers) is the screen gets crunched and I guess it's something to do with being 720 x 480 or something like that. What Fudah says as a solution is to instead go the VGA to Component YUV route as this will not screw with how the image gets scaled. I might know you from the shmups board, so go ask him more about it in the thread I have posted there.