Sounds like there is a problem with your workflow from source to render.
Start with a video standard project setting. 1280x720, 1920x1080, or UHD.
if your photos are not at least the resolution of your project file, the photos will look less resolved.
If your photos are much larger than the project settings, make sure to scale them correctly with pan crop. You can see the scaled resolution in pan/crop>position display. The height and width should not go below your project resolution.
Render to a video profile that matches your project settings. Make sure the render profile>project setting is set to use "video rendering quality: BEST" This will use the better image scaler in Vegas.
Also make sure to use enough bit rate for the changes in your content. Assuming AVC/MP4, for example 16Mbs-22Mbs for HD, or 50-100 for UHD.
The flicker thing with RAYS sounds like it could be RAYS, since RAYS will flicker with changes in the background material. If you want static rays, you may need to render to a an intermediate layer. Rendering to PNG sequence such that you only end up with the text and rays, and the rest of the image alpha. Then you can layer the PNG sequence on top of the other media. Try rays with just text on a black background and see if it still flickers. This is hard to tell what is going on without seeing the composite.