Yeah, I have 4 different 26.8mm posts - two ControlTechs and the Thomson and Orange and I have 3 different diameter measurements for them, the two CTs being almost the same as each other.I can't comment on seatpost issues specifically for the Vitamin T, but I do know stated seatpost sizes can vary from the actual size!
My old Vit-T was fitted with a USE with a shim.
Cheers