ROME (Reuters) -Telecom Italia’s (TIM) top investor Vivendi would only back a sale of the Italian phone group’s fixed network if it adequately valued TIM’s main infrastructure asset, the CEO of the French group was quoted as saying. The comments comes after TIM on Sunday took the first step towards spinning off its landline grid when it signed a non-binding deal with state lender CDP – its second-biggest investor – to create a domestic broadband champion. Under the long-mooted plan TIM would merge its grid assets with those of CDP-owned rival Open Fiber. “I want to be clear on the subject of t…