I am afraid that there is a documentation for this available as of now, but to my understanding the SME will be managing the session.
You can have direct or a turnk through CUCM-SME. SME is used in deployment model for centrailized managment so the SME is the one who would be routing the call in this scenario.
SME should have two trunks one pointing to CUCM and another towards VCS.
However on the leaf CUCM you can have two trunks one configured towards VCS directly and then one configured towards the SME, put this two trunks in the "route list" and assign it to "route-pattern" with higher priority to going towards CUCM-SME.
just in case if the CUCM SME is down then the call should get routed to the VCS directly.