> neoprene has an R value of .15 to .45 /inch and is much more durable,
Very heavy stuff. Very. Try hunting around for some EVA30 closed cell foam: good insulation and much lighter. EVA20 or EVA 15 even, IF you can find it.
> If someone made a double wall titanium thermos I would certainly carry one in the winter!
Glamorous, but not much different from a stainless steel one.
You see, there are three heat-loss paths here: through the vacuum (not affected by the metal), through the stopper (not affected by the metal), and through the thin metal wall at the neck of the flask. SS and Ti are not much different in thermal conductivity, and I question whether you would find the Ti wall much thinner than an SS wall, so the heat loss here is not really all that large. Add to that the fiendish cost of forming Ti into that shape ...
My wife and I LIKE our morning tea stop with a stove and kettle.