Hi
there should be no diff between those platforms in terms of STP
here is the best practice doc for VSS STP in case you've missed something but if it was working on 6509 should be same for 6807
http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Campus/VSS30dg/campusVSS_DG/VSS-dg_ch3.html#wp1079779