A dialect is just a different variety of the same language. They often have different accents and differences in vocabulary (pop vs soda, chips vs fries, etc.)
The definitions for dialects isn’t very solid though. You’ll hear people say a language is just a dialect with an army. For example, Spanish and Portuguese are relatively similar, but are considered different languages. Meanwhile Mandarin and Cantonese are very different, but are considered dialects of Chinese.
Vernacular is a variety of a language (aka a dialect) that the average person uses in a casual context. They way you talk to a friend is vernacular. The way a company writes a PR statement probably isnt.
Latest Answers