Well it's not going to affect the vacuum, since you're only actually improving things. It's the actual contents that presumably are the problem.
Could I suggest (and no, I haven't tried this) that you could do worse than use one with a small-hole pouring spout in the lid. So long as you keep it upright - easy enough in one end of a flight bag I'd have thought, if you leave that open pressures should equalise without excessive loss of temperature. Certainly I've a Stainless steel one I use for hillwalking that would do that admirably.
Or use an old fashioned one with a cork in the top, worst case the cork pops off - but again you really do want to keep it upright ;-), and probably tie a string to the cork !
Just a thought from a simple Engineer who has nothing to do with airliners except sitting up the back with a glass of wine occasionally.
G