Since I don't know this product, I can only answer with (my) common sense:
Air should be a much better insulator than glass, so even if there's no (good) vacuum, the insulation should work pretty well.
One thing to keep in mind is what is mentioned on the Wikipedia article on vacuum flasks
Heat transfer by thermal radiation may be minimized by silvering flask
and observing that a normal vacuum bottle isn't translucent. That means that I would expect some loss due to radiation, since the glass isn't covered in any way.