VSAN 6.2 - 2 Node cluster networking question

Hybrid VSAN6.2 -  2 Node cluster for ROBO

Site A: 2xPhysical Nodes

            - Management Network(VLAN 210)(L3 routable)

            - VM Network (Trunk VLAN 210,220,230,240) (L3)

            - VSAN Network(VLAN450*)(L2 routable only i.e. it'll talk only between these two data nodes)

            *Adding static route from VLAN 450 to VLAN 120 in Site B

SiteB: 1xWitness node

              - Management Network(VLAN 110)(L3 routable)

              - VM Network(Trunk VLAN 110,120,130,140)(L3)

              - VSAN Network(VLAN120)(L3 routable)

              *Adding static route from VLAN 120 to VLAN 450in Site A

Now my questions are all from Site B configuration:-

1. Do I really need to put VSAN on different network(VLAN120), why can't I use the same network (VLAN110) for management as well as VSAN & add the routes to VLAN110 instead of VLAN120?

2. Is it advisable to use one of the VLAN's which we use for VM Network as VSAN Network(As you see VLAN120 is part of VM network as well)?

3. Do we really need to have our network team create a new dedicated VLAN(IP range) created just for Witness VSAN network?

Also let me know if you see any issues in above configuration.

Thanks in advance.