After reading that section of Virginia's law I am fairly certain that the 20 min waiting period only applies to the Intoxilyzer machine, not to a preliminary breath test device like the PBT that you blew into. Under that section it talks about the observation period, the air blank and control sample, the operator's license and the certified instructor...all of these are specific to an Intoxilyzer machine. Under that section of the law the only requirement for conducting the PBT is that it be used in conformity with the manufacturer's operating procedures.
There really aren't many precautions needed for a PBT because by its nature it is not intended to determine an accurate BAC level. The point is mostly to determine whether there has been any amount of alcohol consumption at all. Tobacco could potentially sway the results of the test, but I don't think any amount of tobacco use is going to give you a result other than 0.000 if you hadn't had any alcohol to drink, and all the State has to prove is that you consumed some amount of alcohol.