How much balsamic vinegar? It is listed in the instructions but not in the ingredient list. Also, how much in a 'packet" of orzo?

Rough guide for a main dish is to use 2 oz dry pasta per person. so for a side dish for 6 to 12, use less than that, 1 or 1.5 oz per person...somewhere between 6 and 18 oz.
another way to think about it - pasta often comes in a 12 oz or 1 lb (16 oz) package, so use one of those packages to make this salad.