Thank you for the clarification, for future reference:

It is indeed to use the same soap object for multiple requests, instead of forging a new one for each request.

If you pass "trace" as one of the options in the forge() request, it is considered a default option for the request, while set_option() is a runtime option, and only active for that request (execute call).

So you need to use

$soap = Request::forge('https://api.cvent.com/soap/V200611.asmx?WSDL', array('driver'=>'soap', 'trace' => true));