Is it permissible to use a thermos that is silver-plated on the inside, knowing that I do not drink directly from it, and does this fall under the threat mentioned regarding the use of gold and silver vessels?

The most cautious and blameless for one's conscience is not to drink from a vessel or use it for storing coffee and other things if it contains silver, because some scholars prohibit the use and acquisition of utensils that contain silver. As stated in "Kashshaf al-Qina" and "Al-Rawd al-Murabbi", acquiring and using vessels plated with gold or silver is forbidden due to extravagance, arrogance, and breaking the hearts of the poor.