@missionaryadvocate My metabolism is very fast due to a congenital defect, so despite being considerably smaller and lightweight I sometimes have to go 3000 or above for a couple weeks to gain back weight I'll have lost.
Most fruits and veggies don't give enough calories to straight up bulk with, but they're useful to add to other snacks because they're mostly water and not filling. For example, if you eat pineapple with legumes it adds an interesting flavor, gives ~80 extra calories for a cup, plus the moisture and bromelain helps with the bean digestion.
Like you I eat a lot of nuts/seeds/nut butters - around 85g at minimum - but sometimes I'll have extra nuts and pair it with grapes. Not only is this tastier than nuts on their own, it also adds close to 100 calories from 20 grapes. Might toss an orange into the mix as well, or a cucumber (though that doesn't give much).
I very rarely eat processed food, but when I badly need to bulk back up I'll sometimes pick up a loaf of bread (dark rye) and have a sandwich or two outside of meals. Maybe peanut butter and apple slices, or even something like fake meat with tomato and cabbage. This is an easy way to pick up 200+ calories quickly, but I don't make a habit out of it because I don't like to eat this way.
Potatoes and sweet potatoes are also one of my go-to options, with either steamed potatoes or homemade fries. That might not be an option if you want to keep fiber/volume down though. It depends on how much you can stomach. I can eat a shockingly high amount of food for my size.
Nutritional yeast is a good way to add calories to any non-fruit food you're having. Throw it on salad, beans, potatoes, roasted carrots, etc. It's around 60 calories and 7g of protein for two tablespoons, along with a nice chunk of B vitamins.
Baked tofu is a nice snack option if you have eaten other sources of protein that day. I use extra firm tofu, and usually eat 1/4 of a block (85g serving). It's a very snackable 130 calories if you spice and bake them nice and crispy.
I also freeze bananas and use them to make faux ice cream, which is my most frequently practiced way to pick up calories. It's 1.5-2 bananas blendered with ~0.4cup of whatever milk (usually coconut). The size of bananas determines how much milk, since you don't want to use too much or it'll be watery. At the right ratio it will have a consistency and taste very similar to ice cream. I'll add a scoop of protein powder into it, plus some other frozen fruit for different flavors. With two bananas, a scoop of protein, and maybe some blueberries it can reach 300 calories for a bowl. It's not very filling either, so you can still get dinner in around it.
The challenge I find, and that I'm sure you run into as well, is that on the vegan diet it's difficult to pursue high calories without also getting higher than desired carbs and/or fat. Yeah someone could scarf down pasta and junk food to bulk, but in my experiences you feel like crap after doing that.