Applying mulch is a great way to slow down the growth of weeds and prevent them colonizing your vegetable patch, or indeed, other areas of your garden where they may threaten to choke other plants. They also protect your plants by preventing the soil from freezing in the event of an unexpected frost. It is great that you are thinking ahead and planning what you want to use. As I'm sure you know, mulch comes in two general forms; it can be organic, e.g. bark, leaves or straw; or inorganic, e.g. rubber chips or plastic sheets. Each type of mulch may affect the soil in different ways and it is important to be aware of this when making your selection. Using black plastic sheets warms the soil and while this may be good news for some of your plants, it may stress others that prefer a cooler soil.
The plants you want to grow have different preferences when it comes to soil conditions. Peppers, for example, love warmth and will thrive in a warm soil produced under black plastic whereas lettuce not so much. You also have to take into account what your outdoor temperatures are likely to reach during the summer which I believe is between 60 and 70 F on average in Oregon. If using plastic, you would have to be careful of plants overheating and drying out on very hot days. This can be prevented by using drip irrigation under the plastic.
You could simply use compost spread over the top of your vegetable garden before you start transplanting this year but you would have to ensure that your beds are relatively free of weeds or they will just start to sprout through the layer. Straw is another popular choice in vegetable patches and would be fine to spread around all of your patch. It has the added benefit of providing a good habitat for beneficial insects. You could also consider spreading leaves if you have any composting from the Fall, the drawback is that these will decompose quickly, especially in the warmer weather, and will need replacing throughout the year.
Straw mulch around lettuce seedlings