There are several things you must do to use the Upscale fx correctly, they are outlined in your Vegas Help.

Here is a summary I posted for another user:

https://www.vegascreativesoftware.info/us/forum/upscaling-sd-video--134650/#ca840174

Be aware that as it stands now, using Smart Deinterlace and Upscale require a fundamental change in procedure from using their legacy counterparts:

Unlike legacy deinterlacing, Project Properties must be set to match the resulting output, not the source. For instance

The Smart Upscale filter must be applied at the Media Level, like this. Note that using Sharp Modeling presets require a lot of extra rendering time. I just use a Light Unsharp Mask instead, and live with some softness.

The link to download the latest "VEGAS Pro 19 Introduction to the new features" document is in this post https://www.vegascreativesoftware.info/us/forum/vegas-pro-19-update-5-build-636--135608/

Musicvid and Set. Ah, so you say that one has to apply the Vegas Upscale onto the video (media) before one puts it on the timeline for the slider to work ..is that correct ? ( I am not at my video computer till tomorrow my time, so cannot test that at the moment).
Vkmast. Thanks for the link to the "feature description document". That has very helpful info for all the "new" features.
Thank you Gentleman 🙂🙂🙂

Feature description document from vkmast link above, page 27:

When you use the VEGAS Upscale effect as a video event effect, the Upscale plugin automatically upscales the video to your project's resolution as defined in Project Settings, so this slider remains inactive since you don’t need it. But it does tell you the value of the automatic setting, as you can see in the image. If you use the plug-in as a media effect, the slider becomes active and you can adjust it to whatever upscale level you choose.

Aah ... that explains it. Thanks @set . Oddly, just testing a 600x424 B&W image with Upscale as a MediaFX, the default scaling of 1.00 still looks best.