What Robert and VeganEssentials wrote on the first page really is the truth. When you're bulking, you eat. You cram high quantities of calorie-dense foods together and eat that shiz right up. I haven't bulked since becoming vegan, but my omnivore stuff was absolutely insane. I'd eat a pound of meat and double that in vegetables in one meal. It's that kind of quantity you're looking for if you want to bulk. Bulking is deliberate. It does not happen on accident (except for when you eat half a jar of delicious PB, lol), and that's what I tell people when I'm at work because, seriously, they seem to think that shiz is something that can accidentally pop onto them overnight. Muscle is deliberate, bulking is deliberate. You must deliberately eat lots of food that is high in calories.
Unfortunately this doesn't work for most. The above method leads to huge muscles, but also to huge fat. If your standard intake is 2500kcals and you jump up to 4000, most of the weight you will be gaining is fat....
That all depends on your body, your training, how "clean" you bulk (no bullshit, just whole foods), and your current fat level. While I was doing this, I was losing fat and gaining muscle mass. I was losing weight when I started training (I'm a 5'7 woman and started at 250lbs), but I started eating like this when I was 195 and trying to get down to 160 (weight max for my height/gender in the Marine Corps). I was doing hard cardio, hard lifting, and 1.5 hours of yoga 2-4 times a week towards the end. Took me 3 months to lose 35lbs and I was eating around 3500-4100 calories per day.
But your argument is that jumping from 2500kcal to 4000 overnight will make someone gain fat. Sure. But that doesn't mean that they can't add in 200-400 calories a day for a week, then add in more the next week, and eventually build up to the 4000kcal. I was just illustrating that's what needs to be aimed for. There's a supplement, and I'm not sure if it's vegan, called Black Hole by Controlled Labs. It's an appetite enhancer. I'll check on it.