The big case in this area is Cantrell v. Commonwealth, 229 Va. 387 (1985). This case deals with the issue extensively and lays out a number of rules regarding the involvement of a private prosecutor. Since it is so extensive, I am going to break it down the way I do most cases for my private notes:
Cantrell v. Commonwealth, 229 Va. 387 (1985): (1) Whether a private prosecutor can appear is at the trial court's discretion. (2) A private prosecutor “is absolutely prohibited from (a) taking any position, (b) making any argument, (c) offering any evidence, or (d) advocating any cause which would be forbidden to a public prosecutor.” (3) A private prosecutor cannot (a) initiate a prosecution or (b) appear before a grand jury. (4) The private prosecutor can take part in the case only with the approval of both (a) the trial judge, and (b) the Commonwealth Attorney. (5) The private prosecutor cannot make the closing argument without the approval of the trial court. (6) The private prosecutor cannot (a) take part in a decision to engage in plea bargaining, (b) deciding the terms of a plea bargain, or (c) a decision (i) to accept a plea of guilty to a lesser crime or (ii) to enter a nolle prosequi. (7) The Commonwealth Attorney must remain in control of the case, although there is no specific limit as to how much work the private prosecutor can do. (8) A private prosecutor representing someone with a civil interest in the same circumstance (a) violates Due Process under the Virginia Constitution and (b) requires no showing of prejudice on the defendant's part for reversal of a conviction.There are also several cases which address a bit of the issue. The oldest I found is Compton v. Commonwealth, 163 Va. 999 (1934), in which the Virginia Supreme Court overturned a conviction because the Clerk of Court and High Sheriff, as private citizens, hired a private prosecutor and therefore tainted the case. A few years later, in Commonwealth v. Duling, 79 Va. Cir. 764 (1934), a Virginia Circuit Court laid out the rule that the only way a private prosecutor can participate is with the Commonwealth Attorney's agreement. Of course, this is a non-binding opinion, but it is well reasoned, persuasive, and its position was adopted in Cantrell. In more modern times, in Adkins v. Commonwealth, 26 Va. App. 14 (1997), the Court of Appeals overturned a conviction because an attorney who has been previously hired as a private prosecutor cannot subsequently be appointed as the special (governmental) prosecutor when the Commonwealth Attorney develops a conflict – even if the attorney stops taking money from his clients once appointed special prosecutor. Finally, in Riner v. Commonwealth, 268 Va. 296 (2004), the Virginia Supreme Court held that it was within the trial court's discretion as to whether a conflict existed which would disqualify the private prosecutor (the trial court decided that the private prosecutor did not represent a civil interest against the defendant which would automatically disqualify the private prosecutor).
So, basically, if doubly approved by the judge and the public prosecutor, the hired gun prosecutor can take part in the case as long as he is under the control of the Commonwealth Attorney and has no civil case conflict. In particular, I think the civil case conflict rule is important, although it doesn't entirely eliminate the private prosecutor's monetary inducement to seek a conviction. After all, the party paying for the private prosecutor is paying him to be biased. However, the opinions seem to say that this was okay in Jolly Olde England, so it's okay in Virginia until the General Assembly says it ain't.
The only rule above that I don't think makes much sense is the prohibition against the private attorney making the closing argument unless the trial court approves – effectively creating a presumption that the public prosecutor should do the closing argument. Since the private prosecutor is forbidden from arguing anything which a public prosecutor could not, I don't understand why there is an extra protection added to this specific aspect of the trial. It would make more sense if the public prosecutor was required to make to both the opening statement and closing argument, as a demonstration that he was in control of the case. In fact, since the public prosecutor must stay in control of the case it would make much more sense if the presumption was that the public prosecutor is required to make the opening statement and thus demonstrate his control by laying out the evidence that will be presented. I suspect this is just one of those stray rulings that just keeps getting quoted and relied upon over and over again without any thought put into it.