Then I'm sorry to say it, but your company is in bad shape.
If you allow a vital employee to leave before training someone or developing the right replacement, you're doing it wrong.
Look, it's a 3 party business. If Vidal wants to leave, make him pay the fucking clause and he's free to go. If there's no clause and you have to agree with it, if you don't agree he doesn't leave, because you know, he signed a contract.
General rule of football business. You sell if you are OK with the fee to be received. This is leverage.
Exception cases: you sell even for a lesser fee, in special circunstances where you have less leverage. Ex: Tevez.
Vidal, for all I know, is not an exception, is a common case. You just let the fucker leave IF you do your job and find a suitable replacement FIRST and you agree with the fee. Doesn't look like it's the case here.