Partly depends on dust collector.
Dust collector = high volume low pressure
Shop / household vac = low volume high pressure
If you want dust collection to catch the fine dust (the only bit that matters for health) to then you need a dust collector and to move a lot of air
Dust collectors (HVLP) work by moving a lot of air. So you don't want to restrict that. You want a chip curtain to stop stuff flying away but you don't want to actually have a seal / restrict air coming in to the tool / dust hood.
Most dust collection for CNC routers is totally inadequate for fine dust collection. Often ineffective for the easy (larger) stuff too.
To clear fine dust you need a lot of CFM. That means minimum 4inch duct to dust boot. Smooth ducting is vastly superior to flex ducting. This is difficult due to the moving nature of CNC....
6 inch ducting better