I am curious as to why you would want to substitute fish sauce for soy sauce or lard for anything else.
I wouldn't recommend substituting the fish sauce for soy. They are not really comparable substitutes. I can't see a reason you would want to unless Katherine said, you really like it that much. Fish sauce, if you have used it yet in your asian cooking, is very potent. A little goes a long way. You might consider using the same amount of soy and if you really want to use fish sauce, add a dash to the marinade before cooking. Are you concerned the light soy you have won't be strong enough as opposed to dark soy?
(According to www.foodsubs.com)
Chinese soy sauce comes in light and dark versions. Lite soy sauce has 1/3 less sodium.
Substitutes: tamari (thicker) OR Maggi seasoning OR black bean sauce thinned with water OR kecap manis (much sweeter) OR Worcestershire sauce.
If you just don't like the thought of using lard, keep in mind it is healthier than butter (compare nutrition labels at the grocery store). The only thing I would recommend subsituting is peanut oil, no other oils. First, because the Chinese wouldn't use anything besides peanut oil in a wok, second, the smoking point of peanut oil is the highest meaning you can get it hotter than other oils without burning the oil.