Thanks for your quick response, @walt.farrell!
The spacebar helps in some scenarios.
I also found turning on "Hide Selection While Dragging" (screenshot below) to be helpful.
As of right now, It seems that Affinity Designer currently doesn't have the functionality to turn off the vector selection at all times.
Again, thank you Walt.
Update:
Further working with Designer, I discovered that the blue outline goes away when you select a tool that doesn't use the outline. A tool like zoom (Z) or hand/view (H).
If doing an undo/redo comparison and you would like to not have the blue lines displaying, you can select the zoom (Z) or hand/view (H) tool before toggling through the undo/redo comparisons.