Nobody has any proof that the transaction will be included in 'the next block' just because the vendor has seen it on the network.
At some point the big mining pools will provide a service to merchants. If the transaction carries a sufficient fee, the miners will certify that they have seen the transaction and that they will include it in the next block if they are the ones to generate that block.
That way, merchants can gain "good enough" assurance that the transaction they have seen is "the good one and not the double spend".