MongoDB - Replica sets - Syncing and DC issue
Is this normal behavior during the transition where they have to take themselves offline?


It is expected that your replica set instances will drop network connections when there is a change in primary.


  My only concern in this situation is what happens if VM1a goes down right after LocalMongo comes up? Wouldn't that mean that the only place where data is actually being stored would be on the primary and that it would create a broken link where VM1b (which never went down) wouldn't sync to anything?


Replica set secondaries can sync off of other secondaries, which can help reduce load on your primary. If your secondaries sync source goes down, it will reevaluate and change sync source to an available member.