Consolidate to multiple or single instance SQL Server on a VM with many third party vendor databases? I need to consolidate several older instances to SQL Server 2016 Enterprise on a single VM. Since all of the databases are third party vendor systems, they are a sprawling mess of SQL Agent jobs, custom roles, dynamically created users, and demands for doing installs/upgrades with SA access that I have to fight frequently.

I yearn for the isolation of multiple instances, but hear that they will create resource contention and other woes. In a VM environment where resources such as I/O would be shared across all VMs anyways and being able to set max memory per instance, should I still be avoiding multiple instances? 

Breaking out to separate VMs is not an option for me due to licensing costs. Also, the scale I'm looking at is about 15 different vendor platforms, some of which I know could happily sit in the default instance together without the above-mentioned issues, so I would probably have a maximum of 5 instances if I went that route.
Obviously an opinion here, but...
Personally, I think separate instances is definitely the way to go.  We have several servers at my work like that.  You have so much more control of stuff, especially server related objects like linked servers, etc.  Being able to restart one instance and leave the others alone is a valuable benefit.  And you can upgrade each instance independently as the vendor allows