I was looking for the actual poll to reply to, we don't pre-vac most jobs, and always Pre-spray even if it's light, chemical and dwell is our friend. and find with the teflon glide on the titanium wand, that an extra pass is all that is needed to make up for lack of vacuuming, and it's really easy because of the the teflon we are sliding on.. In actuality to properly vac an entire job, I could do twice as many passes witht eh wand in teh same time, so I have to weigh that.
We generally only vacuum when there are visible chunks, and do find most homeowners do a fair job of vacuuming in our area.
Cali Yank