I'm wanting to cover over my home's old solid wood fascia and soffit probably with aluminum fascia and vinyl soffit. There's an option to have either vented or unvented soffit. Since my current soffit doesn't factor into my attic's ventilation, does vented soffit provide any benefits to the wood/structure underneath?
There would be no benefit other than aesthetic if you are matching the design of another part of your house. The common strategy is to cut out a strip of the plywood soffit before installing the vented aluminum soffit. This would allow you to use the airflow in your attic ventilation.
A vented soffit can greatly aid in the ventilation of your attic. The idea is to draw cooler outside air into the attic to help exhaust warmer humid air out top. If you only have vents near the top of your attic (ridge or gable vents) it will be beneficial to add vents in the soffits. Besides improving attic ventilation this will reduce the force sucking conditioned air out of your home and into your attic.
But keep in mind there needs to be some path for air to get from the soffits to the open spaces of the attic. If you have loose insulation stuffed into the corners or mis-designed vaulted ceilings there may be insufficient air space. There are stiff boards you can install in the corners to ensure a proper air space.