If marinating, then it's best to do so after grilling. The cell walls contain over 90% moisture, so a wet, salty, and acidic solution will draw out the water, break down the structure, and make the vegetable mushy if not completely drained and dried. To flavor the surface, it's better to use oil and dried seasonings first. Add dressing or drizzle with balsamic vinegar, red wine vinegar, or citrus, such as lemon juice, while still warm to brighten the taste before serving.