Place rosemary needles and cucumber slices in a cocktail shaker, and muddle to release cucumber juice and rosemary oils. Add infused vodka, lime zest, lime juice, simple syrup, salt and ice. Shake until well-chilled (about 30 seconds). Gently stir in sparkling water, then taste for sweetness. If it's too tart, stir in a bit more simple syrup. Double strain (through the cocktail strainer and a fine mesh strainer) into an ice-filled glass, and garnish with a cucumber round and rosemary sprig.