Few things ...

On Vmware world you usually install MS-SQL on its own virtual machine or single Windows Os. That gives you more flexibility and the ability to isolate performance.

Now, going back to your initial question, if I have VMware machine A, and another one called B, each running its own instance of MS-SQL, they should NOT "fight" for resources or affect the others performance; that's one of the virtualizations premises. But that is only true if the VMware admin properly configure the Hypervisor that hosts those virtual machines. If there is not enough hardware (enough hosts, RAM, CPU, slow SAN, etc) machine A will start having issues because machine B needs some vCPU or RAM in order to process a query.

Above is the main reason to get a VMware expert that also posses some MS-SQL expertise.