Createa a VersionControlServer instance..
Then call the GetItem method to obtain the instance of the file you want to download, then call its Download method.
E.g.
TeamFoundationServer tfsServer = new TeamFoundationServer(tfsUri, credentials);
VersionControlServer vcServer = (VersionControlServer)tfsServer.GetService(typeof(VersionControlServer));
if (vcServer == null)
throw new ApplicationException("Invalid vcServer");
Item i = vcServer.GetItem(@"$/Path/To/MyFile.File");
i.DownloadFile(@"C:\MyFile.File");
I just noticed you're doing this in VB.Net, should be easy to convert from C#.