Oftentimes Vietnamese people pair two synonyms or two words that are similar by association for added emphasis or to transform it into a noun.
e.g. bơi and lội both mean to swim but bơi lội means swimming.
e.g. nhanh and lẹ both mean fast/swift/quick but nhanh lẹ is more emphatic.
In the case of xây and xây dựng, xây = to build/construct and dựng = to erect/to place upright. You would say "chuyên ngành xây dựng" for the construction industry, however, to ask "what are you building?" you'd use "bạn đang xây cái gì?"
As a native Cantonese speaker, the characters helps me a lot in my learning process. Not only the Sino-Vietnamese readings, but native chữ nôm also helps me remember a lot of native Vietnamese words, because they hint the pronunciation and the category of those words. After every lesson on Duolingo, I put all words I learned into this spreadsheet and check their characters whenever it's available. Hope this helps!
The contents may not all be correct though. Please comment if you find anything wrong! The category is also just based on my intuitions so there's no need to take it too seriously.