VS is in "Stand In" the mode for getting live responses but it is not routing to LIVE system properly for all the transactions.
When a VS is in "Stand In" mode the request is sent to the LIVE system in the VSM-->Live Invocation step.
Live Invocation step needs to be configured with LIVE system information like hostname, port, and the base path (Replacement URI, if needed). If the Replacement URI is configured then all the transaction will use it as a basepath and this might cause issues if it is not same for different requests.
Do not add anything in the VSM-->Live Invocation Step-->Replacement URI field unless as needed. The VS request base path is going to be sent to LIVE system by default.