In a medium sized bowl, whisk together the fish sauce, lemongrass, pepper and sugar. Add the shrimp and stir to coat. Marinate shrimp for 10 minutes. In meantime, boil a pot of water. Add vermicelli and cook for 4 minutes. Drain immediately, rinse with cold water, and drain again. Try to remove as much water as possible. Set aside.
Grill the shrimp in a large skillet with 1 tbl cooking oil on high heat until cooked through, about 2 minutes. You could also skewer the shrimp and grill on your outdoor grill. When shrimp is cool enough to handle, bisect each shrimp down the middle of its back so that you have two identical halves of shrimp. This makes the shrimp thin enough to wrap evenly.
Grab a pie plate or rectangular baking dish and fill it with about 1" of warm temperature water. Dip one rice paper round in the dish for a quick 2-seconds. Lift round and let the water drip off. Place on clean, dry work surface. Blot the top of the round with a paper towel. You'll work one roll at a time.
Now it's time to roll. The wrapper will still be a little stiff. By the time you finish piling on the ingredients, it will have softened enough to roll. Lay 4 halves of the shrimp in a line near the bottom 1/3 of the round. Top with some lettuce, vermicelli noodles, carrots, bell pepper, mango, and finishing with 2 mint leaves. Try to keep the ingredients compact and piled on top of the shrimp. Starting with the side closest to you, roll up the roll tight, stopping halfway to gently tug back on the roll to tighten. The wrapper is self-sealing. You can use a sharp knife to cut off the two ends to make it look neater.
Transfer to platter and cover with plastic wrap to prevent drying. Wipe counter or plate dry and repeat with remaining.
Some rice rounds are made thicker than others. If you find your wrapper is a little too stiff by the time you need to wrap, either blot less water off the rice round next time, let it sit for a few seconds before adding ingredients or dip in just slightly warmer water (but not hot!) Just remember to let the water drip off and wipe your working surface dry each time - a dry surface allows the rice paper to stick to the surface and create a little tension so that you can wrap.
Also, I've found it's best to work one roll at a time - since it only takes a few seconds for the rice round to soften, you really aren't saving a lot of time by dipping all your rice rounds at once. In fact, if 2 rounds touch each other - they will stick and you'll have a heck of a time getting them apart without tearing or wrinkling. If you must dip all at once, separate each wet round with a damp paper towel.