Do VMMC MBBS students have secured seats for PG in VMMC?

No, VMMC MBBS students do not have secured seats for PG in VMMC. Here is how the internal quota of the university works

Once the students complete their UG course from their college, they're eligible for the internal quota of PG seats of their University and not the specific college. This means that the internal quota is for the university and not the college. In your case, the internal PG quota belongs to IPU and not VMMC. So, Students of all the colleges under IPU are eligible for IPU PG seats. 

Earlier there was only VMMC (UG) under it, Mao PG seats were easily available in VMMC and RML, so the seats were kind of secured. But with more and more colleges under IPU, the PG quota of IPU too has lost much of its value.

So, the VMMC MBBS students don't have secured PG seats.