You answered your own question: " In reviewing rulings, it appears that the subtraction is only for the original amounts withheld for retirement plans and any increase or growth is subject to tax." The retirement amounts subtracted from Virginia income tax would only include "after tax" contributions previously taxed by Pennsylvania.
Any earnings that grew tax free within the account are taxable to Virginia when you take the money out, because you never paid PA income tax on it.
PA has a unique tax system however Ruling § 58.1-1821 Application: Individual Income Tax is applicable.
You can hire a local tax attorney who handles Virginia audits.
