Starting with VxRail 4.7.300, you can use Layer 3 between the two sites, but you must use Layer 2 for the management network.
For vSAN, you can use either Layer 2 or Layer 3. However, for Layer 2 configurations. Ensure that the Witness site in L2 enclosures is using high bandwidth and low latency.
Connectivity between the data sites and the witness must be Layer 3. Figure 1 illustrates a supported configuration.
Figure 1. VxRail Supported Topology