I just read the entire manual for pre 1988 Vilgant There is no option to reduce the flue collar requirement like the Encore. It is clearly states
the 8" requirement. and do not reduce it in the connector run. Reading the suplemental part of the manual, in the pages 20 and beyond,
it does point out venting into an 8/8 clay liner is aceptable.
Please explain your current flue and venting setup.
Did you see Struggle's finished product photo damn it looks brand new
how old is the villigant you are looking at.
After reading the VC pre 88 manual they provide a good description of secondary burn, thermotatically control, and horrizontal burning
man these mid 80's VC stoves had the Everburn technology back then, the secondary burn and draft chamber paths.
Little know info: In anticipation to the EPA 1990 requirements VC started the research sooner than many other manufactures
Even before the EPA mandates, the state of Oregon imposed similar requirements, which eventually mirrored the EPA
In the ladder part of 1985 VC tested and certified their stove to the Oregon standards These are the same stoves designs that passed the 1990 EPA
requirements. For all practical purposes, the Oregon tested certified stoves are EPA compliant. they achieved the 63% effeciencies and the grams discharge requirments
Read this
http://www.vermontcastings.com/catalog/elements/files/Defiant_Vig_Res_Intre_Pre88-0226.pdf