How to dissolve salt into mashed potatoes
Until the salt is dissolved, you will always have the problem of separate grains. It seems that your potatoes are not moist enough for it to happen on its own. And salt won't dissolve in fat. 

My suggestion is to choose a liquid - and it can be water, if you insist on staying vegan, else dairy is the typical choice - and dissolve the salt in it. You don't need much, a teaspoonful may be enough. Once you have the salty liquid, mix a small amount of potatoes into the liquid until dispersed. Then add a bit more, repeat, using larger and larger portions of mashed potatoes, until all is mixed. This will give you an even dispersal of the salty taste.