I learned how to make Vietnamese spring rolls from a friend who grew up in Saigon. Paul's recipe used the meat from fresh crabs - he wouldn't even consider using the canned stuff. Picking the meat out of fresh crabs is a lot of work, so over time, I changed the recipe, and I now use minced pork and fresh shrimp in place of the crabmeat.
Paul's most interesting advice was to use canned soft drink as part of the dipping liquid; he says the sugar in the drink caramelises as the spring rolls are fried, giving them an attractive brown colour. Don't use drinks with artificial sweeteners - they have to include sugar. If you like, just dissolve some sugar in water - it has the same effect. Adding the soft drink (or sugar) to the dipping liquid makes the spring rolls stick together as they're frying, which is a bit of a nuisance, so if you prefer, just use plain water.
If you can find them, use Vietnamese rice papers, instead of the thicker, more brittle Thai ones that are easily available in most supermarkets. Vietnamese rice papers are thin and pliable - you can fold them and they won't break, and when fried, become very crisp and delicate. If you can't find them, the Thai rice papers are fine, too.
I double fry the spring rolls. The first frying is to cook the filling; the second is heat them, and to crisp up the exterior. You can fry them the first time in advance and leave them at room temperature for a couple of hours (less if it's very hot in the kitchen). Fry them a second time just before serving them.
Put the mung bean vermicelli and cloud ear mushrooms in a bowl and add warm water to cover. Leave to soak until soft, then drain in a colander. Squeeze out the excess water from the mung bean vermicelli then use scissors to cut it into shorter lengths. Squeeze the excess water out of the cloud ear mushrooms then finely chop them.
Thinly slice the onion and mince the garlic. Coarsely grate the carrot. Use a paring knife to peel the water chestnuts, then rinse them well before roughly chopping them. Cut the shrimp meat into small pieces.
Put the pork in the bowl and add onion, garlic, shrimp, carrot, mung bean vermicelli, cloud ear mushrooms and water chestnuts, then mix thoroughly. Mix in the eggs, then season with the fish sauce, sugar, pepper and about 1/2 tsp of salt and combine thoroughly.
If using the soft drink, pour it into a wide bowl and add 300ml (1¼ cups) cool water. If using the sugar, pour it into a wide bowl, add 600ml (2½ cups) of cool water and stir until the sugar dissolves.
Lay a clean, dry dishcloth on a work surface. Dip a rice paper in the liquid and immediately lift it out before laying it on the cloth; the paper will quickly become limp and pliable. Spoon some of the filling on the lower third of the rice paper. Fold up the bottom of the paper to cover the filling, then fold in the sides towards the centre. Press gently to squeeze out as much air as possible, then roll to the opposite side so it forms a tight cylinder. Continue with the remaining rice papers and filling.
Pour cooking oil to the depth of about 1.5cm (⅔in) in a skillet placed over a medium-high flame. When the oil is hot (170°C/340°F), start frying the spring rolls in batches. Fry the spring rolls for a few minutes, turning them over as necessary so they brown evenly. If they stick together, gently pry them apart. Drain them on paper towels and fry the remaining rolls.
For the second frying, heat the oil to 180°C (350°°F). Fry the spring rolls briefly, just long enough to crisp and brown the exterior. Drain them on paper towels.
Make the sauce. Finely mince the garlic and the chillies, and coarsely grate the carrot. Mix the fish sauce with the granulated sugar and stir to dissolve. Mix in the lime juice and rice vinegar, then adjust the seasonings to taste. Dilute the sauce with a little filtered water, then mix in the garlic, chilli and carrot.
To eat, wrap the spring rolls in lettuce leaves and add the herbs. Dip the parcel into the sauce (or drizzle it on top) before eating.