Poison Oak Control in Forest
First of all, I would suggest limiting your poison oak control to areas along trails, roadways, and other places where people are most likely to be exposed to it. It is not practical to control poison oak over a large area without also damaging other vegetation.
If you are going to apply herbicides, please consult the following publications which discuss which chemical ingredients are most harmful to grapes, and also how to minimize drift by choosing the right spray method and timing:
Are Your Weed Control Products Damaging Nearby Vineyards?
Preventing Herbicide Injury and Drift to Grapes
The below link from the PNW Weed Management Handbook lists several products and rates that are effective on poison oak. However, you'll note that grapes are also sensitive to them.
Timing the application for late summer, when grapes are less susceptible, but before the poison oak leaves start to change color, is a good strategy. Also apply in cool and still conditions, with a ground-based method like a backpack sprayer to minimize drift. These are basic suggestions, but I strongly recommend referring to the above publications for more detail.
Thank you for your quick and helpful answer.