A VPS is a replacement for a cheap dedicated server. A VPS is a 'piece' of a high end enterprise server. A cheap dedicated server is just that, cheap and unreliable. Its hard disk will fail and your business will be down for a few hours/days.

A VPS won't fail, but it might get slow. Happily you can upgrade it with an email, phone call, or mouse click.

When your site has very high volumes, thats the time to look at a real dedicated server...redundant power supplies, hardware RAID, offsite backup etc. Thats an expensive game.