I have had that black preview problem A LOT years ago too. Unfortunately I don't remember if there was a fix for it.
My assumption is the system or software runs out of resources and throttles by ignoring footage.
Check your RAM & CPU resources by going here: Vegas / Options / Preferences... / Video tab
Then just give it a lot RAM capacity & as many CPU threads as your chip has.
In Vegas' Project Media tab from the default GUI, there's a thunder symbol at the top left which says "Remove All Unused Media From Project" when you mouse-hover over it.
Maybe hit that every now and then during the editing process to have a clean conscience about nothing unnecessary taking resources.
I'm sure the glitch fade you mentioned is something like this at 3:19
https://youtu.be/XRxrp1DYupU?t=3m17s
I remember ending up rendering videos in several lossless pieces to prevent those glitches. Afterwards I merged them back together in a new project file.