How to resolve IPSec Site to Site VPN issues if you have two vendor products like Cisco Router at this end and Juniper Router at another end ? Option

The best way to resolve issues between multivendor devices is to do a packet capture and log messages which appear in your syslog server or device logs.

You can capture the traffic by setting up SPAN/RSPAN or port mirroring for the ports where Cisco and Juniper devices are connected, when you are setting up IPSEC tunnel or using it.99% of the times, you can resolve IPSEC tunnel issues by checking and googling the logs which appear during IPSEC setup, be aware that if you want debugs, then it is not a good idea for devices in production.

Hope I answered your question.