It depends. Some wineries may allow you to wander through a portion of their vineyard, and some may request that you remain near the tasting room property.
Wineries are very concerned about the spread of pests (such as Phylloxera), which can be carried on your shoes, and can be transferred from one winery to another. Throughout Oregon’s winegrowing history, pests and diseases have destroyed acres, even entire crops, so be sure to ask the staff at the winery before you go wandering among the grapes.
If you are permitted to walk in the vineyards, you are in for a real treat! Few things can compare to holding a wonderful glass of wine in your hand, as you stroll past the very grapes that make that wine possible!