Most Americans get more protein than they need, and the occasional low-protein dinner really never hurt anybody. That being said, if you look at vegetarian recipes and think "where's the protein?" or you have particularly high protein needs (pregnant? raising teenage boys? work a physically demanding job?), then you might want to have a few tricks up your sleeve for adding protein to a vegetarian dinner.
Beans - Oh, how we love beans in my house. They can be added to vegetarian soups, stews, and pasta sauces. They make wonderful side dishes on their own or combined with vegetables. You can mash almost any bean and turn it into a spread, which is particularly nice in summer. (See recipe below.)
Cheese - Now and then my mom used to put a bowl of ricotta cheese on the table. We would each add a big spoonful to our plate of pasta with tomato sauce. A little grated cheddar or monterey jack is delicious on vegetarian chilli. When I make vegetarian soups, we often have bread and cheese on the side. In the summer, bread, cheese, and salad or cooked vegetables can be a complete meal.
Eggs - While my husband is not a fan of breakfast for dinner, he never says no to frittata. (See recipe below.) Eggs can also be beaten and mixed into very hot, freshly cooked rice.
Tofu - I mostly use tofu in stir fries. There are lots of great recipes out there for sneaking it into chilli and tacos though.
You might notice the one thing missing from this list is fake meat. We very rarely eat fake meat. I ate more of it when I first became a vegetarian, but the more I read the labels, the more it fell into the category of "processed foods" for me. Once I started eating meat a couple of times a week, I more or less lost all interest in fake meat as I was getting my full of meaty goodness from the real thing. That being said, protein crumbles can be very good in chilli, and the occasional fake chicken nugget with tater tots can take you right back to grade school. (I sometimes wonder if the cafeteria chicken nuggets weren't mostly fake meat anyway.)
White Bean Spread: Saute a little garlic in olive oil. Add two cups of cooked white beans (or a can of beans, drained), salt, a little water, and sage. Mash the beans. This is good either warm or cold. I generally mound it on a plate or shallow bowl and garnish it with chopped tomatoes or olives. A little olive oil drizzled on top is particularly good if you serve it warm.
A Basic Frittata: Saute a little garlic in olive oil, add a cooked green vegetable (asparagus, broccoli, spinach, or kale), and eggs beaten with a little salt and pepper. I generally make about a half cup of cooked vegetable per person and two eggs. If you are using leafy greens, make sure to squeeze the extra water out of them. Let the eggs cook without mixing them until browned on bottom. If the top is still too moist for your taste, you can flip it in the pan or stick it under the broiler, assuming your pan is broiler safe. A little grated parmesan or pecorino is nice on top. Crumbled feta is good too. We generally eat this with a loaf of bread and salad.