Make the dipping sauce. Add all ingredients to a bowl in order listed and mix with a fork until smooth OR add all ingredients to a blender and blend until smooth. Add more water to achieve a thinner consistency.
Prepare all vegetable and other fillings FIRST! Have all your ingredients in separate piles on a cutting board nearby.
Dampen 2 medium sized, clean dishcloths and lay them flat on your work surface, beside each other. Fill a large bowl of water (wide enough to fit an entire rice paper) and place it near your damp towels.
Take one of the rice papers and immerse it in the water. Hold it in there for 15-20 seconds. The rice paper will still feel a little hard but after you remove it from the water it will gradually soften. Lay it flat on one of the damp dishtowels.
In the middle of the rice paper place 1 Boston lettuce leaf. Then place 2 pieces of red pepper, 2 pieces of mango, 2 pieces of avocado, 2 pieces of cucumber and one tofu strip in or adjacent to the lettuce leaf.
The rice paper should be fully softened by now. Take the edge closest to you and fold it over the filling, wraping it tightly. Now fold the sides in towards the center of the rice paper, over the previous fold and hold them down firmly. Finally, while holding the folds in place, roll the entire spring roll forward.
Place the spring roll on the OTHER damp towel and fold the towel over it to keep it from drying out.
Repeat this process with all other rice papers being sure to mix up the ingredient combinations!
Once all the spring rolls are made, place one at a time on the cutting board and cut them in half on a diagonal. Place them on a serving tray cut side facing up and serve with the vegan spicy tahini sauce.