Thermos: No, too heavy! worst comes to worst, you can carry a Nalgene and put some boiling water into it, and stuck it into sleeping bag if your are totally freezing.
Down Vest: i agree, but it could be a good winter additional layer to boost your system. especially in high aerobic activity when putting parka is too warm (sweat) but just a synthetic shirt is too cold (core is cold). could be also good when you stop and take a break.
in any case, you will get cold. it is just a question of when, and how you manage it. suggestion: get out in the winter for a weekend and camp with someone you trust, get frosty (but not hypothermic, of course!) after this, you'd have the experience with REAL cold and when it gets cold in the shoulder season you'd know what to expect and you'd know your limits, you'd stretch out your "cold tolerance muscle", so to speak. you'd also find out what works and what doesn't for you. we can make hundreds of suggestions, but you only know how your body reacts, so test your layering system before you get out there.