The 58 Mbps quoted is the maximum speed that your phone line (or cable connection) can handle. However data can't come into your computer any faster than the server sending it can pump it out.
When you use a BitTorrent client, such as Vuze, you're downloading data from the computers of other people on the internet, so the download speed you can get is limited by the upload speeds that they've got.
Further, many ISPs deliberately slow down BitTorrent traffic (or even block it altogether), as
(a) it takes resources from other users ; and
(b) such traffic is almost always illegal.