Could we just clarify this point? That Vodacom broadcasts the rain network ID using its tower and antennas? I have a local tower that has had zero physical changes since rain started. The tower ID 77603329 exactly matches the one seen when using a Vodacom sim. This suggests rain is merely piggybacking off Vodacom's physical infrastructure in this case.
I have only seen it once on a Vodacom tower before where Rain uses the same sector antennas as Vodacom. I was suspecting but obviously can not confirm.
Even then Rain does not roam on Vodacom and the network is not shared unless I have a different definition of roaming.
For me roaming means network a does not have towers, network equipment and the customers of network A connects to network B's network using the same spectrum and same technology.
If Rain shares a tower or sector antenna the equipment and network backhaul will still be theirs because they own spectrum in B41. No one else can have that spectrum.
I have done some some tests. On your Vodacom simcard. Insert it in a B618 and force B38. Okay it works. Now force the Rain sim to B38. Okay cell id matches. Vodacom roaming on Rain because we know for a FACT that Vodacom does not have spectrum in that band.
Now force both simcards to band 1. Only the Vodacom one should work and the Rain simcard won't. It means the Vodacom sim is on it's own network and we know for a FACT Rain does not own spectrum in band 1.
This is a very easy trick to test the roaming and I wil be VERY stunned if the Rain simcard works on Band 1 or Band 8 which vodacom has LTE on.