I can't remember I used a pixelate effect any time, but used other effects. Some of the effects render dependant on preview (render) quality. Sharpen is one of these effects.
If you set preview to best/full, theoretically you should see exatcly what you will see when rendered, while with simply
"preview/full" not necessarily.
So this is just a shot in the dark, but maybe set preview to best/full, and adjust the effects properties so.
Other question, that you render to the same resolution as it is the project properties?
I'll give that a try. When I rendered again the other day, I noticed quite a bit of difference between Vegas 9 and 12. We usually edit on 12, but sometimes we need to edit in 9, because 12 chokes on the files by a certain client.
I think I need to add extra large pixels because when the image is scaled to a smaller output frame size, some of the blurring effect disappears. It doesn't happen as much ith v12.