Here, apparently, the opinion of an expert is not so important as the presence of precedents. There are a lot of vegan bodybuilders and athletes on the internet. Perhaps in Russia there are no such numbers, but if you google vegan athletes, vegan bodybuilders and so on, you will find a bunch of materials, tips, youtube channels and other useful information.
By the way, in sports food stores soy protein is quite common in Russia.
You can, but you need to understand that the diet needs to be properly composed. For example, Alexey Voevoda, an athlete, has titles in armwrestling and bobsledding, and looks impressive, and at the same time is a vegan (you can watch him on the program Evening Urgant: youtube.com), although he made up his diet himself, but guided by smart books and scientifically proven results.
I'm not a nutritionist, and I'm not a bodybuilder, but I think you can gain weight. For every 1 kg of weight, a person needs to consume about 1-2 grams of protein, and he, in addition to meat, is contained in eggs, as far as I understand, vegetarians can eat them. Also in dairy products. Plus, there is a huge selection of sports nutrition that will help with weight gain. Accordingly, it is quite possible to gain weight.