NTP uses the concept of a stratum to describe how many hops (routers) away a machine is from an authoritative time source, usually a reference clock. A reference clock is a stratum 0 device that is assumed to be accurate and has little or no delay associated with it. Stratum 0 servers cannot be used on the network but they are directly connected to computers which then operate as stratum-1 servers. A stratum 1 time server acts as a primary network time standard.

VPN 0 - Transport VPN, which carries control traffic via the configured WAN transport interfaces.
Initially, VPN 0 contains all of a device's interfaces except for the management interface, and all interfaces are disabled.
VPN 512 - Management VPN, which carries out-of-band network management traffic among the Viptela devices in the overlay network. The interface used for management traffic resides in VPN
512. By default, VPN 512 is configured and enabled on all vEdge routers except for vEdge 100.
For controller devices, by default, VPN 512 is not configured.
VPNs 1 through 511, and 513 through 65530 - VPNs on vEdge routers for service-side data traffic.
https://www.cisco.com/c/en/us/support/docs/routers/12000-series-routers/47321-ciscoef.html