He was wearing a vest __his shirt

你好，新鲜百分百
针对你的两个疑问，分点回答，稍为繁复，希望多一点参考，帮助你消疑解惑。
1.He was wearing a vest under his shirt.
填空介词“under”或者“underneath”.
特参考于某外文网站，保证正品。
例：Do you wear a short sleeved vest underneath your shirt?
2.while跟when 区别
①when意为"在……时刻或时期"，它可兼指"时间点"与"时间段"，所引导的从句的动词既可以是终止性动词，也可是持续性动词。如：
When I got home, he was having supper.我到家时，他正在吃饭。
②while只指"时间段"，不指"时间点"，从句的动词只限于持续性动词。如：
While I slept, a thief broke in.在我睡觉时,盗贼闯了进来。
③从句的谓语动词如表示状态时，通常用while。如：
We must strike while the iron is hot.我们应该趁热打铁。
④when从句与主句动作先后发生时，不能与while互换。
When he has finished his work, he takes a short rest.每当他做完工作后，总要稍稍休息一下。(when = after)

outside

while 在……期间
when 在那时