Line the bottom of a 9 x 13 inch baking pan with parchment paper.
Place the butter in a large (6 quart or larger1) pot.
Separate the marshmallows, setting 2 cups aside in a bowl or container and placing the rest into the pot with the butter
Place the pot over medium heat and slowly heat the butter and marshmallows, stirring very frequently. The butter should melt and then the marshmallows should begin to melt.
Continue heating and stirring the ingredients until the mixture is smooth and fluffy.
Remove the pot from heat, then stir in the vanilla and salt.
Stir in the crispy rice cereal and the reserved marshmallows.
Press the mixture into the prepared baking pan2.
If using sprinkles, distribute them over the mixture at this point and gently press them into the top surface so they stick.
Let the vegan rice krispie treats set at room temperature for about 1 hour.
Slice into 20 bars and serve.