I was forced to rehose the entire vacuum system with only two available sizes, 4mm and 6.5mm inside diameter. For the most part I found that the reduced inside diameter did not restrict the lines enough to affect the engine or emissions systems however I had to reuse use about 10" of "middle sized" hose that fit neither of the two new diameters I have but it was mainly only used between the Sol-Vac VsV junction (there's a reducer just before the junction, actually) and the vacuum source.
While driving city/highway and idling I was not able to observe any issues with the new hose diameters, so it should be okay.