Finally I've finally solved it but unfortunately after I submitted my coursework, the solution is pretty straightforward, I am puzzled on how you guys could not figure it out.
Dim con As New SqlConnection(_conString)
Dim sql As String = "SELECT AccountType FROM tblUser WHERE Username=@Username"
Dim cmd As New SqlCommand(sql, con)
cmd.Parameters.AddWithValue("@Username", txtUsername.Text)
Dim myReader As SqlDataReader
con.Open()
myReader = cmd.ExecuteReader
DetailsView1.DataSource = myReader
DetailsView1.DataBind()
Dim AT As String = myReader("AccountType")
If AT.Contains("Customer") Then
Response.Redirect("~/Customer.aspx")
Else : Response.Redirect("~/Seller.aspx")