What solution does Juniper provide to monitor vpn users log in?

Juniper can use SNMP v2 or v3 for monitoring so we can use native console or any SNMP management tool.

on SRX you can use following command to check the users:

show security dynamic-vpn users

Junos Space 

NetScreen-5200 is a2-slot chassis integrating firewall security, VPN security, traffic management functionality, Denial of Service, and Distributed Denial of Service protection, delivering up to10 Gbps of firewall throughput.