Oh yes, it is good in lettuce; all sorts of other things are good in lettuce wraps too, incl. curry type stuff I find that I eat less in wraps than with a Big Plate O Curry in front of me.
Nuoc cham (my preference for summer rolls, too) is just something like:
one part fish sauce
one part (or less) lime juice, pref. fresh squeezed
one part water
sugar to taste
minced chiles and garlic to taste
optionally add a little crushed peanuts and/or finely shredded carrot
It also makes an untraditional but excellent salad dressing