Is the D attorney allowed to have a prospective juror do the FST during VD? How do I prevent it?? Is there caselaw?

Just make sure the juror is sober.

Of course, you could object (as defense attorneys often do during trial) that the defense attorney is not certified in SFSTs.

It is an obvious attempt by the defense to present actual evidence that a jury might have to consider during trial, seeking to get a commitment from the juror about the evidence. No different than putting on evidence of an actual breath test during voir dire. Ask the judge to have the defense save it for trial.

It is also an attempt to convince the panel that sfst's are impossible to do (coming up famous quote) even when sober. They often want to introduce the panel to their view that sfst's are not accurate and are really designed to make those taking them look intoxicated when they are not; this is when their hero did poorly on them. Such a demonstration forms no basis for real questions for the panel to aid the attorney in selecting the jury (always remember that is the purpose of voir dire.) It's a trick to subvert the reason for VD and to commit the panel and present unsworn evidence without cross examination before the trial begins. And, by the way, the panel members are not the ones on trial, Dufus is.