Yes, it should be fine. I’ve used that and similar cables purchased off eBay, custom and home made cables and there has been no difference in appearance – that I can tell. I think with the Tomee, the comments regarding poor image quality, is primarily due to the device they are connecting to is displaying the VESA 640×480 aspect ratio and not the correct 480p for the Dreamcast. VGA improves the definition and clarity of the image and because the Dreamcast uses a fixed dithering technique on its output this shows up more evidently. When viewing VGA in VESA the dithering can become distorted making the image appear to have vertical lines running through it.
In your OSSC be sure to select 480p in sampler and have H.active set to 640. I also have H.backporch at 111 and V.backporch at 33 (you may need to use slightly different settings) to correctly center the image. To help get the perfect image I actually use GDMenu as the fixed dithering is more pronounced and visible, if it looks straight and an even pattern the image is exactly as it should be. Take a look at this screenshot…