Nicotine is legal because it was never made illegal, similar to most things which are legal.

Most of the time in which nicotine has been available it has been through combustion which destroys some of the nicotine and will cause a newcomer like you to cough like a maniac and vomit long before you've taken in enough nicotine to faint. Now that vapes designed to be easy to inhale are available and the amount of nicotine can be adjusted to be higher or lower by the manufacturer, a newcomer can inhale much more nicotine at once than has been possible until very recently. Also, the experience you just had is very unlikely to happen again for two reasons. First, you're probably not going to take huge hits of that vape anymore and second, your brain has now been exposed to that chemical so you now have some tolerance.