Why did V only wear a steel breastplate instead of something more bulletproof like Kevlar?

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.

Why did V only wear a steel breastplate instead of something more bulletproof like Kevlar?

1. From a director's perspective, having V use a steel breastplate for armor subtly reinforces a parallel between V and Guy Fawkes, of whom would have also worn a steel breastplate back in the 17th century if he had found himself in a similar situation.

2. Even if V did wear a Kevlar vest, chances are he still would have incurred a significant number of serious wounds to then die in a similar fashion. A Kevlar vest doesn't cover your entire torso, it often leaves the low half of your abdomen exposed, as well as your arms, legs, neck, etc. Only unless V were wearing something like a full body / juggernaut kind of suit would he have stood a good chance of surviving.

3. It could also be argued that if V did wear something like full body armor then it would have significantly impeded his mobility, flexibility, etc., and he'd have a much more difficult time eliminating Creedy and his men. So in that regard, V put confidence in his own abilities over Creedy and his men's.

• After thought: Evey wasn't apart of V's original plan and if he had never crossed paths with her in that alley, my guess is he still would have sacrificed himself after killing Creedy by getting on the train with the explosives to ensure they successfully reach their destination. That being said, my main believed-to-be reasoning behind why V wore a steel breastplate instead of something like Kevlar is because of the Guy Fawkes parallel.

This is a question that mostly can be answered with a not really great question...

Why make the movie at all?

This isn't a great answer obviously. But it's typical film stuff in any combination of these tropes.

1. Death is Dramatic
2. Show must go on or But now I must go
3. Inspirational Martyr

It's mostly just a way to get rid of V, him to profuse his love for the main actress and us to feel sad, but happy his end goals finally succeeded and the corrupt government is retaken by the people.

V was also a product from an old and corrupt time. He cannot continue to exist anymore and he likely knew it.