There are many layers to this topic, regardless if it is personal or commercial clients. As a wedding photographer I would ask my lawyer to include in my contract that the couple confirms a model release for my own commercial use(that can be assigned to third parties), and that photos may be provided to vendors. In practice, it is obviously better to ask the client directly rather than to depend on the contract.
State privacy laws vary significantly, but they generally place the liability on the direct publisher of a photo of commercial use. So if you publish the photos in advertising it is your liability, and not your liability if the vendor does the same on their own. However, this may not prevent an angry couple from suing you as well as the source of the photos. A smart vendor would negotiate use of the photos, a model release, and a gift as compensation as part of their own contract with the couple in advance of the event.
As always, it is smart to understand business issues through peer advice, but only rely on your lawyer and insurance company for contract language. Since your business insurance provider may cover you for these types of issues, it is best to include them in your strategy.