Should I have my old ceiling vacuumed before installing insulation?
You can get simple lead test kits at your local hardware or box store. Check the "dust" for lead. If you don't feel comfortable testing it yourself, hire a pro to do it.  Try not to disturb the dust until it is determined to be lead free. Lead is only bad for you when airborne, so keep it in place, don't disturb it. Wear a dust mask or respirator to be safe.  If it is lead contaminated, do not use a vacuum to collect it. You will need to contract a lead remediator to do this with specialized equipment. Do NOT attempt to do this yourself or hire a non licenced person to remove it.  If there is no lead, you can remove the droppings and dust with a shop vac or hire it done.  Any no-lead dust is not big deal, but I would be inclined to remove any rodent droppings before installing your insulation.