I usually render to MPEG 2 DVD ARchitect NTSC - I've been rendering these for YouTube and DVDs - until about 2 weeks ago, it worked fine - then all of a sudden every image I import renders at very very low res
One issue you might be having, if I understand, is your images are 4 or 5 times more resolution than your video project. In this case, you are relying upon Vegas to reduce your images. In my experience, this is not its strong point. I usually have my images be around 2 times larger than project in order to allow for zooms or moves. Anything bigger I will use a photo program to reduce resolution.
With the information you provided your project resolution is too low. VP resizing images to project resolution and then upscaling it to your render template resolution. [Edit: this is wrong, see @Marco.'s comment below]
If you use the same project to render twice -- for DVD and for YouTube, then you need to increase your project resolution (Alt + Enter) to 1920 x 1080.
"VP resizing images to project resolution and then upscaling it to your render template resolution."
This is a common false assumption. Actually Vegas Pro will not resize your source image size until the render process, no matter which project size is used.
Thus you can use a gigapixel photo such as 60000x20000 pixel, put it into an 8x8 pixel project than render to 4k. The result would be exactly same as if you had used a 8192x8192 pixel project (even if you had used FX like Unsharp Mask, Sharpen, Pixelate, no matter if the FX is used as Media FX, Event FX, Track FX, Master Bus FX or Output FX).
@Marco. Not sure why I was tagged in your response. I said nothing about being changed to project resolution. What I said is I don't trust a video program to resize my photo, especially when it is resizing an image that is 4 or 5 times larger than project resolution (which in my case is 90% of the time the final rendered resolution). I would rather resize it in a photo program to around 2x so Vegas has only minimal resizing efforts required when rendering.. I have not tested to see which gives the best resizing, just giving my thoughts.