'Vainglorious' describes someone who is far too proud of himself — of what he can do, of what he has done, of how important he is — and who shows it. It is stronger and older-sounding than 'proud' or 'boastful'. The picture behind it is of glory that is empty: a man dressed up in his own achievements, making sure everyone sees, when in truth there is not much inside the costume. That emptiness is the whole point of the word, and it is why 'vainglorious' has a sad, disapproving edge that 'confident' does not. This is a literary word. You will meet it in books, in history writing, and in serious newspaper columns describing a leader, a general, or a plan that was more about the leader's pride than about anything useful — 'a vainglorious war', 'a vainglorious speech'. Nobody says it at dinner. In real conversation you would say 'he's so full of himself' or 'he's such a show-off'. Keep 'vainglorious' for when you are writing, and when you want the reader to feel that the pride is hollow. The noun is 'vainglory'.

自負虛誇的、好大喜功的（書面語，帶「其實沒什麼料卻愛擺架子」的貶義；口語會說 he's so full of himself）