我想通过DOS命令或者用vb写个程序把指定文件上传到公网指定的服务器。

用Inet控件吧，他可以帮助你完成你想实现的功能。
设置信息：
Inet1.Protocol = icFTP
Inet1.RemoteHost = ServerName'服务器地址
Inet1.RemotePort = 21'FTP端口
Inet1.UserName = uName'用户名
Inet1.Password = uPWS'密码

下载文件：
Inet1.Execute "", "GET 1.txt d:\1.txt"
ri = Inet1.StillExecuting
Do While ri
ri = Inet1.StillExecuting
DoEvents
Loop
上传文件：
Inet1.Execute "", "PUT d:\1.txt 1.txt"
ri = Inet1.StillExecuting
Do While ri
ri = Inet1.StillExecuting
DoEvents
Loop