Don't feel like you have to use one long piece of grapevine across the mantel. It is much easier to cut segments from a wreath and arrange them individually. Allow curled pieces to drape off the sides, but for safety, keep them well above the firebox. If desired, cover the mantel surface with fabric or felt to prevent scratches.
Candles positioned low near the grapevine must be enclosed in lanterns or glass holders. Place containers on tiles, cork coasters, or trivets to protect the mantel from heat. (If you have some great tall candlesticks, arrange them at the ends. Stagger small flowerpots along the mantel between the tall candlesticks.)