Your question is based on the presumption of wanting to live/survive. V simply doesn't fit in that category.
V fits the narrative trope of a scarred victim whose very existence is torture to himself, every single day. He is motivated to keep going for a single purpose: to get revenge on those who scarred and tortured him. The movie's title specifically points out that he's only out for revenge, it is the sole pillar on which his "V" persona is built.
As a result of the showdown, V has killed the las two remaining two members on his kill list: Sutler and Creedy. There is no one left for him to wage his vendetta against. He no longer has a mission.
And he's not looking for a happily ever after. Regardless of objective truth or not, he genuinely feels like he cannot live a normal life anymore, and the life he does lead is agony to him. The only thing that kept him going all this time was his vendetta, his desire to punish and avenge.
He went to that showdown knowing full well that he wouldn't come back (cfr the goodbye scene with Evey). That wasn't just an educated guess, it was the actual plan. Which is why he didn't bother with more armor - there was no point to it.
In a way, he had already fallen down the proverbial well - he just managed to hang on long enough to take the others down with him. V, Sutler, Creedy, Prothero, and the rest of them, are all part of a bloody past that should be erased to make way for a brighter future (led by Evey). V himself is too spiteful to be part of that future, let alone try to shape it. His only role was to take down the fascist regime, not to rebuild the subsequent peaceful regime.