How to enable RST log specifically for one particular virtual server ?

You can do packet sniff to know where the RST came from

user complains issue is intermittent, we have no clue when issue appear again. i want to monitor the traffic for long periods especially reset logs.

High speed logging profile for all access requests off to a Splunk server or similar. ?

There is no high speed logging enabled, how it can be done specifically for virtual server ? we have a logging server to log all messages

You can simply use SOL13223 to log RST and then grep the logs for specific information.