HI Sam,
If you haven’t already, before I spent any more time running this down, I would do a full program reset…..
While holding down the ‘Shift’+’Ctrl’ keys, double left click on the Vegas Pro desktop icon. In the window, check the option to ‘Delete all application cache data’. This will reset Vegas to it’s original default settings and configuration and delete the application cache, which often causes Vegas to behave strangely.
If that doesn’t fix it, I would go to whatever FX you applied before the missing FX and turn them off, one at a time, render, look at results! Depending on how complex your project is, this could be time consuming, obviously.
Because you can render out the missing FX and it works, it would point to me something before it is messing things up.
Regardless, whatever information you can provide about your system and FX chain you are using, might help other members point to the cause or suggest a fix.
Best Regards……George