Why did Thackeray subtitle Vanity Fair "A book without a hero"?

There seems to be an inescapable conclusion to make
regarding the subtitle of this excellent Victorian classic, and that is that the
overarching theme of this novel is the way in which all of society and all in it are
shown to be overwhelmingly vain. Some may be more vain than others, but the subtitle
indicates that none, according to the author, can be called a
hero.

It may be useful to recognise that the title is an
allusion from Pilgrim's Progress and refers to a city, called Vanity Fair, whose soul
purpose was to divert and entertain people with pleasures and goods and distract them on
their way to reaching heaven. Thackeray places Vanity Fair in
London during Victorian times, and presents us with a range of middle and upper class
characters who only live for achieving greater wealth and higher social status, and show
themselves willing to cheat, lie and deceive in order to reach those goals. Thus it is
that Thackeray gave his novel the subtitle that it has: the pages of this novel are
remarkable for the complete absence of anyone who matches up to the definition of the
hero. All are self-serving characters.