Step #1 To make the dressing: in a small mixing bowl, mix the vinegar, salt, mustard, & pepper; whisk until the salt is dissolved.
Step #2 ADd in the tarragon & slowly whisk in the olive oil; set aside.
Step #3 Use a pan with a steamer basket; add water to the pan to reach just below the basket; bring water to a boil; add potatoes to the basket (they should fit in a single layer; if you have more than one layer, steam the potatoes in batches; if the potatoes are very large, cut them in half & steam them cut-side down).
Step #4 Cover up the pan & steam until done, checking to make sure the water does not boil away (small potatoes--15 mins; larger potatoes--18-20 mins).
Step #5 When done, transfer potatoes to a colander to drain; remove basket & pour out water from pan.
Step #6 Return potatoes to the warm pan; toss over low heat/flame to dry them.
Step #7 When cool enough to handle, cut into 1/4-inch thick slices.
Step #8 Place potato slices In a large-ish bowl; sprinkle with vinegar while still warm, then let cool to about room temp.
Step #9 Add the zucchini, squash, bell peppers, sugar snap peas, & onion to the potaotes.
Step #10 Pour the dressing over the mixture (stir the dressing again just before adding to the potatoes); toss carefully.
Step #11 Serve as soon as possible.
Step #12 Note--to blanch the vegetables, fill a large bowl or pan with cold water & some ice cubes.
Step #13 Fill a saucepan 3/4 full w/ water & bring to a boil.
Step #14 Drop in the vegetables & boil for 1 mins; drain & as soon as possible immerse the vegetables in the ice water; when cold, drain & pat dry.