Should I have the old ceiling vacuumed before installing insulation

You can do a hot roof and not require any ventilation, though I would not recommend it anywhere snow accumulates on shingle roofs. Even if i don't like it, it is allowable.

Whether you need additional ventilation depends on how much you are getting now and what is allowed by the governing building code for your area. According to the International Building Code for One and Two Family Dwellings, R806 Roof Ventilation, you need the equivalent of 1/300 of the area ventilated in free vent area. If the soffit vents do not provide this, you need to install additional ventilation or opt for the hot roof concept.

Also, the attic area needs to accessible by a min 22x30 inch opening. You must also install a class 1 or II vapor retarder on the warm side of the insulation or the ventilation requirements double.

The building code does not address moisture control within the garage space, it's only concerned with moisture in concealed spaces or general ventilation of habitable spaces. You may wish to address moisture in the garage itself with exhaust fans or operable openings. This is unrelated to attic ventillation requirements.

Remember, heat rises. In the summer the second floor will be hotter than the first for several reasons. I found that after reaching a reasonable amount of insulation in the attic which helps more in the winter season, removing the heat from the attic is the best remedy. I installed a power vent running off a thermostat. You need to make sure you have enough vents around the eve of the house and that they are not blocked for this to work effectively. The cooler outside air will replace the hot attic air and as hot air rises, this happens quickly. It worked for me when I lived in New York, but now that I am in Florida it is even more important to get the heat out of the attic. Most Florida homes have wind turbine vents and several of them to make this effect work. Newer homes here have high ceilings eliminating attics.