I think the content of the stream that you post is not correct, you need to ASCII encoded the string before adding it to your request.
See the modifications to your code here:
Dim postString As String
postString = "use...=<>&pass...=<>"
Dim postData As Byte()
postData = Encoding.UTF8.GetBytes(postString)
Dim myReq2 As System.Net.HttpWebRequest = System.Net.WebRequest.Create(roServerSettings.GET_MSI_URL & "Login.aspx")
myReq2.Method = "POST"
myReq2.ContentLength = postData.Length
myReq2.ContentType = "application/x-www-form-urlencoded"
myReq2.CookieContainer = cookies
myReq2.KeepAlive = True
Dim sw As System.IO.StreamWriter = New System.IO.StreamWriter(myReq2.GetRequestStream())
sw.Write(postData)
sw.Close()
This should get the string correctly encoded to the other side.
If you still get an error, check, if you are sending correctly formatted name-value pairs as per the API specification.