I don't understand this,
I have a 9:16 image 1440 x 2560, I've added Boris's Bokeh Lights, playback is rubbish as with a lot of Boris's Sapphire filters in Vegas,
Build Dynamic Ram works fine but is sometimes a little slow, so i keep trying Selectively prerender but in this case it reduced the width to 1440 x 2160 adding black bars or in other projects it can do other unpredictable 'odd' things,
Removing all prerenders in Tools - Clean up prerenders returns the image back to correct
PS, Render to a new track works fine, but i get the pop-up option of aspect/size etc. with that