Change the preview setting to Best (Full) and click on the little disk icon above the preview window.

Right-click on the Video Preview and pick "Display at project size" . Also set preview quality to best.

Set your project property "Interlace" format to "progressive" and the (advanced property) "Deinterlace method" to "Interpolate" so you won't get any interlace artifacts in your images. Make sure to set the interlace property back before you render your project, you only want progressive for the still capture step.

Use "Save Timeline snapshot to file" because it deals with pixel aspect ratio, while "Copy Snapshot" does not.

All of the above will work but (as with most things in Vegas) there are other ways to skin a cat.

I never see a difference between Best(full) and Good(full). My understanding of "Best" is that is that it more rigorously renders effects (uses bicubic interpolation while good uses biliear). Best will do a great job with oversized stills but can reduce playback performance a bit.

Temporarily setting a project to progressive and interpolate will also reduce playback performance. Not that that's bad if you're just doing a frame grab session but I find that I like to be able to move quickly through the timeline.

I use "copy snapshot" daily. A typical day for me involves making 30-60 frame grabs-usually of circuit boards and finely detailed parts. I have an "action" set to a key command in photoshop. The command makes a new file, pastes the clipboard, and resizes the image to 655x480. I find this gets an image into Photoshop faster than using Vegas to save to file.

There's not a lot of movement to my shots so I usually don't need to deinterlace. When I do need it I often just draw a marquee around the object and deinterlace only that. For a horse show you probably need to deinterlace the whole frame because everything will be moving.

As you do frame grabs in Vegas you should drop markers so you can find the frames again. If you like you can save a version of your project as "HorseShow_for_Stills_Export.veg".

BTW, Using Vegas to "save as file" degrades the still image. I see this most with stills of motherboards where the pinholes of a cpu socket will be totally obliterated. If your goal is to use the still as a freeze frame in Vegas you'll get crisper results by copying to clipboard, pasting in photoshop, saving as psd or png, and then dragging back to the media pool. You need to make Vegas treat the 720x480 still as 0.9091 pixel aspect ratio. I have vegas set up to treat all PSD images this way.

Rob Mack