Add potatoes to a very large sauté pan and cover with 2 inches of cold water. Place pan over high heat and bring to a boil; season with 1 teaspoon salt, reduce heat to medium and simmer until potatoes are nearly fork-tender, about 15 minutes.
Add cauliflower florets and cook, covered, until fork tender, 5–7 minutes. Drain potatoes and cauliflower; let cool.
Meanwhile, prepare dressing in a large bowl.
Once cool enough to handle, tear potatoes into bite-sized chunks and add to dressing. Add drained cauliflower, celery, chives, lemon zest and vinegar. Toss to combine then season with salt and pepper to taste.
Vinegar: you can use just about kind of vinegar, we like white distilled, white wine, red wine or salad vinegar. If you want this salad to be uber-creamy, use 1 pound of potatoes instead of 1 ½ pounds.