There is no option to throttle specific VPG. However, you can set the priority on VPG which should allocate bandwidth to VPGs based on the priority set when there is limited bandwidth.
Here is some information regarding the priority – Determine the priority for transferring data from the protected site to the recovery site when there is limited bandwidth and more than one VPG is defined on the protected site. When there are updates to virtual machines protected in VPGs with different priorities, first the updates from the VPG with the highest priority are passed over the WAN. Medium priority VPGs will only be able to use whatever bandwidth is left after the high priority VPGs have used it. This is also true between medium and low priorities.