OK. Just did a bit of a search and found Loving v. Commonwealth, 1966, 206 Va. 924. Once you get past the fact that it's a case about "miscegenetic marriages" it provides what seems to be the standard for the use of banishment in Virginia.
Apparently, banishment is allowed in Virginia. However, "the defendant's background or the circumstances of the case [must] indicate that [banishment is] necessary to secure the defendant's rehabilitation and to accomplish the purposes [of the statute]." If it isn't necessary the banishment is unreasonable. Such unreasonableness renders the sentence void and [it] will, accordingly, be vacated and set aside."
Hmmmm . . . That's a standard applicable to judicially imposed banishment. If banishment was part of a plea agreement the defendant could stipulate the need. It would be an interesting option. Instead of 3 months in jail an agreement to banishment for 3 years. That might be a win-win for all sides.