Based on the story:
Almost five thousand years agone, there were pilgrims walking to the Celestial City, as these two honest persons are; and BEELZEBUB, APOLLYON, and LEGION, with their companions, perceiving by the path that the pilgrims made, that their way to the City lay through this town of Vanity, they contrived here to set up a fair; a fair wherein should be sold of all sorts of vanity, and that it should last all the year long. Therefore at this fair are all such merchandise sold: as houses, lands, trades, places, honours, preferments, titles, countries, kingdoms; lusts, pleasures, and delights of all sorts – as whores, bawds, wives, husbands, children, masters, servants, lives, blood, bodies, souls, silver, gold, pearls, precious stones, and what not.
And moreover, at this fair there is at all times to be deceivers, cheats, games, plays, fools, apes, knaves, and rogues and that of every kind.
Here are to be seen, too – and that for nothing – thefts, murders, adulteries, false-swearers, and that of a blood red colour
Taken from this, you can say that Bunyan understands that Vanity is a test of human weakness. If, in order to get to Celestial City, you have to cross Vanity, it is implied that our weaknesses and limitations for all that is not neccesary nor important will be put right in front of the passers by, and therefore all only the victorious will be able to get to Celestial City.
According to the reading, it is implied that vanity is universal, and a human malady, more than a specific sin. It is more than sin; it is a capacity that we all have and must resist. It enables all over evils to come through, and produces every possibility for further weakness.