Addressing (2) per http://akanda.io/tip-of-the-week/tip-...

While GRE and VXLAN seem like equivalent encapsulation methods there is a subtle difference which can impact your deployment if you’re utilizing equal-cost multi-path (ECMP) routing.  VXLAN utilizes UDP, so nearly all routers properly distribute traffic to the next hop by hashing over the 5 tuple that include the UDP source and destination ports.  

GRE tunneling is a bit different since IP packets do not contain the information necessary to construct a 5-tuple hash.  How the router selects the path for a GRE packet is largely dependent on the hardware as some routers will use the GRE tunnel key to help create the hash.  If you intend to utilize ECMP in the underlay with GRE, it is important to select hardware that supports both otherwise you might not get the performance and utilization you expect.

In general very good answer is provided by https://ask.openstack.org/en/question...

VLAN vs (GRE or VXLAN) tunelling

However, regardless VXLAN ( been standard for RDO deployments) comes with no problems via dumb switches, when VLAN requires expensive tunable switches with trunk ports to connect Compute Nodes with Controller/Network Personally , I have noticed that no matter of L2POP && ARP Responding been setup for RDO VXLAN deployment, switching to VLAN ( with growth of expense of network hardware ) make RDO deployment network communications faster and just a bit more reliable. English is not my first language so I ,actually, want to say that "IF"

1. Number of tenants << 4095 ( VLAN limitation 12 bits header )
2. Client is convinced to pay for Cisco Switches ( trunk port supports)
3. Details here :-

http://www.cisco.com/c/en/us/td/docs/...
I would do RDO Mitaka (Liberty) ML2&OVSL&VLAN deployment.