It is necessary to wear underwear for a warm vest. It's uncomfortable to wear directly, and the warmth effect is not good, and your arms will be exposed.
Thermal underwear can be worn close to the body, but it is not recommended, because the thick thermal underwear sold in the market is usually made of a variety of composite sandwich materials, and a fluffy chemical fiber or ultra-thin film is sandwiched between two layers of ordinary cotton cloth, which can prevent human skin from exchanging gas and heat with the outside world, thus bringing warmth. The thermal underwear made of this material will rub the skin and bring discomfort, and may also reduce the moisture of the skin, causing discomfort such as dryness and itching, so it is better to wear ordinary autumn clothes inside the thermal underwear.
I suggest that when you choose clothes in winter, you should choose light and warm clothes, which will make you more comfortable and relaxed.