Add a Request Data Manager DPH after your SOAP DPH in your listen step.
For each argument you want to use for matching with transactions in your .vsi add a "Keep" action in the RDM DPH.
Make sure that your .vsi now only has valid signatures, i.e. only signatures with the arguments you want to keep; all the arguments that the RDM DPH will remove from your incoming requests also need to be removed from the transactions in your .vsi.
Cheers,
Danny