VGFacts

Fan theories suggest that Vault Boy isn't giving a thumbs up or winking. Vault Boy is using an old war pre-strategy that suggests if a bomb's cloud is smaller than one's thumb, while the arm is fully extended, they are outside of the radiation zone.

http://www.falloutfacts.com/other/fallou...story.html
https://m.reddit.com/r/FanTheories/comme...ly_giving/
http://gamerant.com/fallout-vault-boy-thumb/
https://www.inverse.com/article/10099-va...ar-fallout

https://imgur.com/gallery/vtexofP

Can't say this one is true.