I"m not sure what you call this, but being able to see the FX and composite option without adding the effect. being able to mouse over the option is so much faster then adding and playing back. (the mouse pointer doesn't show)
Since most of Vegas's codebase is outdate codes from the Sonic Foundry era, performance is poor.
So trying to implement such a feature is likely to have a more negative impact on Vegas's existing performance.
@lan-mLMC Capcut (the tiktok editor) works a similar way, but where it's different is you have to single click before it does something. I show this by mousing over a line of transitions and nothing happens, then I go the line above that I had previously clicked on, and they work. I don't know if capcut actually does precache, but Vegas working that way could give it the opportunity to cache and be capable of giving a good preview.