In a small pot combine currants, vinegar, and a pinch of sugar. Bring to a simmer, then remove from heat. Cool to room temperature and whisk in oil; season with salt and pepper.
Meanwhile, steam cauliflower until crisp-tender, about 10 minutes. In a large bowl, combine cauliflower, onion, celery, parsley, and currant mixture. Season with salt and pepper and toss to combine.