I see this now, but obviously didn't see it when applied originally.
Didn't catch it until spotted reviewing the render.
Went back and found it like this on the timeline.
Dunno why it's appeared now.
In order to get through a complete render, I've had to disable GPU acceleration, which was set to use NVIDIA during the edit. Perhaps editing with GPU acceleration when applied, but then switching it off later brought out the glitch?

Update: I re-enabled NVIDIA acceleration and restarted Vegas, and the dimensionality fx worked properly again.
I re-rendered the section on the timeline, spliced that sub-render into the glitched full render, and am re-rendering the whole.
A lot of workarounds :-(