Step #1 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 #2 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 #3 When done, transfer potatoes to a colander to drain; remove basket & pour out water from pan.
Step #4 Return potatoes to the warm pan; toss over low heat/flame to dry them.
Step #5 When potatoes are cool enough to handle, peel if desired, then cut into 1/2-inch cubes.
Step #6 Place the cubed potatoes In a large-ish bowl; sprinkle with vinegar & salt while still warm.
Step #7 Add in the lamb.
Step #8 To make the dressing: mix together the first 7 dressing ingredients in a small bowl.
Step #9 Whisk in the olive oil; pour the dressing over the potato-lamb mixture.