By default, vSphere provides a mechanism that creates scheduling queues for every virtual machine file. Each file, for example .vmdk, gets its own bandwidth controls.
This mechanism ensures that I/O for a particular virtual machine file, such as .vmdk, goes into its own separate queue and does not interfere with I/Os from other files.
This capability is enabled by default. If you need to turn it off, you can do this by adjusting the VMkernel.Boot.isPerFileSchedModelActive parameter in the advanced system settings.