I have a suspicion that this is either a hardware related bug or Vegas purchased vs Vegas cracked bug, because it’s not something I’ve been able to recreate and I’ve only encountered it with different system specs.
Had this happen a very, very long time ago and what (I think) temporarily fixed it for me was simply zooming the timeline in/out quickly with the mousewheel. That or slightly shifting the timeline back/forth – Shift + mousewheel. Could be totally wrong though.
If you’re using an nvidia card – have you tried switching to the latest studio drivers?
+I’m wondering – does this also occur when you edit the selection by grabbing and moving the yellow triangles of a deselected region that isn’t looped? So make some arbitrary selection, click anywhere else in the timeline so the region is no longer selected, then try dragging the yellow triangle. You can then toggle the looping with Q.
Added an image for what I mean by “deselected region” / “region is no longer selected” – not looped, no vertical lines.
I feel like if it doesn’t render any lines on the sides, it should also not render any artifacts.