It depends on why you have a different machine per vendor. If it's for security isolation, separate instances (one license) should be adequate. If it's for DR/BC, it's probably not doing what someone thinks it is (I've seen this argument before, so I mention it). If it's to simplify support, or to comply with vendor licenses/contracts, then it may not do what you need.

If I'm not mistaken, Enterprise Edition of SQL Server can cover multiple virtual machines with a single license. If you want specifics, call Microsoft and get their official answer on this. You may have specific circumstances that modify how the licensing would work, and it's best to get the answers directly from the source.