Deutsche Bank has upgraded its rating for Vodafone from 'hold' to 'buy', saying that a takeover of Kabel Deutschland (KDG) could push the British telecoms group towards potential value crystallisation.In a research report on Friday, the broker said: "We think a KDG deal, consistent with our convergent thesis, is likely the tipping point for a Verizon Wireless (VZW) deal. Leverage is now high, and VOD needs to preserve liquidity for its European turnaround. "Furthermore there is value maximisation to consider with US competition getting tougher. With a deal looking more probable and consensus having rebased after a tough H2, we upgrade to 'buy'."The broker said that a VZW deal would be the best option for Vodafone to "crystallise valuation and preserve liquidity as [it] continues its European turnaround".The broker has tweaked its target price up from 216p to 217p after making minor changes to forecasts.The stock was up 1.2% at 188.58p by 10:14 on Friday.