I've no idea what Soylent is, but I think I could eat vegetable soup every for the rest of my life and be happy. But it has potatoes in it and I have a potato problem. Probably not a ton of protein in it though.
Discussion
It's a vegan soy-based meal replacement. A lot more sustainable to buy in bulk, which I do, but I buy these bottles to keep in the mini fridge at work. Even after drinking it for the last 10 years I am happy to buy more regularly. It is not the entirety of my diet but easily over half.
Could kick up the protein in your vegetable soup with chickpeas, lentils, wild rice, chia/hemp seeds (if you're into that)..
Some others could be spinach, sweet potatoes, and asparagus. Possibly quinoa on top.
Nutritional yeast is popular. I use it sometimes but can't say I love it just yet. Haven't cooked with it much but it seems to be used frequently in some recipes.
Lots of good options!
https://www.healthline.com/nutrition/protein-for-vegans-vegetarians