vegan dipping sauce for summer rolls?

I would go with soy sauce, toasted sesame oil, scallions/chives, ginger, minced chili and lime juice....maybe a pinch of allspice.

thank you everyone. I will try these and report back over the next few weeks

For the rice wrappers: at a local Burmese restaurant I watched the women making them and instead of immersing the dried wrappers in water they sprayed water from a spray bottle until they were pliable. This way they are not as wet and somewhat easier to deal with.
For the dipping sauce I agree with others who suggest peanut sauce, which can easily be made vegan and still flavorful. Or if you want to keep it a light, you could make a Chinese dumpling dipping sauce. To me the key is to use red or black (chinkiang) vinegar, which are much more flavorfull that white rice wine vinegar. 50:50 vinegar:soy, add any of the following: thinly sliced scallions, crushed garlic and/or ginger, chili-garlic paste, sesame oil.
If you're trying to replicate a Vietnamese dipping sauce the ingredients of nuoc cham (besides the fish sauce) are usually lime and sugar. The brilliant technique I learned for making it, and making sure the flavors are balanced is from VietWorld Kitchen. http://vietworldkitchen.typepad.com/blog/2008/11/basic-vietnamese-dipping-sauce-nuoc-cham.html First make limeade and taste it for sugar/sour balance. Then add the fish sauce (or liquid aminos, for vegan), and other additions like crushed garlic and chili peppers, or pounded fried shallots.

I found a fish sauce substitute, but haven't tried it yet. It was from food52 cook monkeymom. for 3 Tb fish sauce use 1 Tb soy sauce, 1 Tb lime juice, 1 tsp brown sugar, 1/2 tsp salt, 1 tsp vegetarian worcestershire sauce.
There was a recent foodpickle question asking for dipping sauces. You could check that, and also check recipes for spring rolls as well as summer rolls. And ask your local restaurant -- they might be able to fix up a substitute sauce for you.

If you think it is the fish sauce that is the flavor you're missing (which it certainly could be because it's pretty central and very unique), you could try using a bit of Bragg's Amino - a fermented soy product - or a bit of miso, to see if you could get a similar umami effect.

I'm not sure- I can't really identify ingredients in prepared foods. i'm hoping it's not fish sauce. I'll try some of these things out. I guess it's hit or miss since I can't say what I'm looking for. I think it's a little sweet too.
I'm not being picky- just looking for what the neighborhood joints offer for dipping sauce as a starting point. There must be some standard recipe they all use.

The Restaurant sauces have ginger (probably grated fresh) in them. Is that the flavor you're looking for?

what do they serve in restaurants? it's a thin sauce and dark brown. do yo think lemongrass is the flavor i'm missing?

You could try making a vietnamese peanut dipping sauce which usually consists of hoisin (store-bought or homemade), peanut butter, rice vinegar, garlic and chili.

how about a sweet and sour tamarind sauce?
http://www.food52.com/recipes/10460_sweet_sour_tamarind_sauce