What food items are very healthy and have the fewest calories?

A plant based diet is best.

Carrots, cabbage, citrus fruits, berries of all kinds, strawberries, blackberries, raspberries, melons, lowfat or non-fat yogurts, chicken, fish, tuna fish, hard boiled eggs, leafy green vegetables, on & on. Dieting or not the body needs a good amount of protein and healthy fats likes nuts and avocados.

Well. Celery is a negative calorie food. That is, it takes more calories for your body to digest it,than the calories taken from the celery.

Warning. Over-consumption can lead to health problems.

As @Coloma points out, a well balanced diet is necessary, meaning proteins, and healthy fats.

But if you are looking for vitamin dense without a lot of calories, you would be hard pressed to find better than cruciferous vegetables:

Arugula, Bok Choy, Broccoli, Brussel Sprouts, Cabbage, Cauliflower, Collard Greens, and Kale.

I could almost live on marinated cucumbers in rice wine vinegar. You can eat gallons of them. I also love cabbage prepared in all ways, raw, baked, steamed, and make a quick lazy persons cole slaw. Shred a bunch of cabbage and drizzle in a bit of miracle whip and rice wine vinegar mixed together.

Which vegetables are ones with high enough protein and least amount of calories?

Vegetables do not supply complete protein. To create a complete protein with non-animal foods like milk, cheese, meat, yogurt, eggs, you must combine certain plant foods. Beans and rice, other legumes and beans/rice, or corn, but there are no, actual, vegetables that have more than trace amounts of incomplete proteins. For example split pea soup with cornbread would form a complete protein the only kind the body can truly utilize to maintain the building blocks of life.

@Coloma Thanks, I was thinking plant food not vetetables. How about lentiles?

As an aside, why is there a distinction between split pea and just regular peas?

Yes, Lentils are a good source of protein when combined with other grains.
Sweet peas are the pea variety used fresh and Garden peas are the peas that are dried to make split peas that need cooking to re-hydrate.

Here is a list of high protein legumes and grains for eating alone or combining for maximum amino acids that form a complete protein.

www.greatist.com/health/complete-vegetarian-proteins