Generally, you don't need to marinate most vegetables before stir-frying; quick cooking over high heat is usually enough to bring out their flavor and keep them crisp-tender.
Unlike meats, which benefit greatly from marination to tenderize and infuse flavor, most vegetables cook so quickly in a stir-fry that a marinade doesn't have much time to penetrate. Their natural flavors, combined with the stir-fry sauce added at the end, are usually sufficient. However, there are a few exceptions where a quick marinade can be beneficial:
Instead of marinating, focus on adding your vegetables to the hot wok in stages, according to their cooking time, to ensure they all cook perfectly without becoming soggy or mushy.