EVPN/VXLAN fabrics are widely deployed these days, and therefore, the need for troubleshooting vtep-related issues is increasing. 
Here is the basic starting point for data collection when encountering vtep interface issues.

When you encounter any vtep-related issues, collecting these information will be the first step for the troubleshooting process.

1. Check the underlay interfaces, if any of them experienced frequent flapping or showing instability in BGP sessions

2. Configure any any level syslog for gathering syslogs in all levels, for better understanding on what is going on 

set system syslog archive size 20m
set system syslog archive files 10
set system syslog file any_messages any any
set system syslog file any_messages explicit-priority
set system syslog time-format year
set system syslog time-format millisecond

3. Configure traceoptions for protocol EVPN and l2ald process as below

set protocols evpn traceoptions file evpn.log 
set protocols evpn traceoptions flag all 
set protocols l2-learning traceoptions file l2ald.log 
set protocols l2-learning traceoptions file size 50m 
set protocols l2-learning traceoptions file files 10 
set protocols l2-learning traceoptions level all 
set protocols l2-learning traceoptions flag all 

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4. For some latest Junos releases, the "request support information" has additional options for collecting feature-specific data. Collecting both basic RSI without option, as well as feature-specific RSI will greatly help troubleshooting the issue.

{master:0}
juniper@SW> request support information ?
Possible completions:
  <[Enter]>            Execute this command
  all-members          Generate in-depth system status report for all virtual chassis members
  brief                Display brief output for selected commands
  cos                  Generate in-depth cos status report
  ddos-debug           Generate in-depth ddos status report
  dhcp-debug           Generate in-depth dhcp status report
  dot1x                Generate in-depth dot1x status report
  evpn-mpls            Generate in-depth evpn-mpls status report
  evpn-vxlan           Generate in-depth evpn-vxlan status report
  l2-debug             Generate in-depth l2 status report
  l3-debug             Generate in-depth l3 status report
  lag-debug            Generate in-depth lag status report
  local                Generate in-depth system status report for local virtual chassis member
  member               Generate in-depth system status report for specific virtual chassis member (0..9)
  mpls-debug           Generate in-depth mpls status report
  multicast-debug      Generate in-depth multicast status report
  security-components  Generate Report for Security Components
  stp-debug            Generate in-depth stp status report
  |                    Pipe through a command

5. For releases that does not support feature-specific RSI, collecting the below list of operational commands will be helpful identifying vtep related issues.

show evpn database | no-more
show evpn instance extensive | no-more
show interface vtep statistics
show ethernet-switching context-history | no-more
show ethernet-switching evpn arp-table | no-more
show ethernet-switching mac-learning-log | no-more
show ethernet-switching mac-ip-table | no-more
show ethernet-switching vxlan-tunnel-end-point esi | no-more
show ethernet-switching vxlan-tunnel-end-point mh-peers | no-more
show ethernet-switching vxlan-tunnel-end-point remote summary
show ethernet-switching vxlan-tunnel-end-point remote | no-more
show ethernet-switching vxlan-tunnel-end-point remote mac-table | no-more
show ethernet-switching vxlan-tunnel-end-point source | no-more
show bgp summary | no-more
show bgp neighbor | no-more
show ospf neighbor extensive | no-more
show ospf route extensive | no-more
show ospf database extensive | no-more
show ddos-protection protocols statistics terse | no-more