I have tried your code and its working fine in the vb.net.(Assuming the drop down is available in ASPX) Is the dropdown
RegionList
is available in your aspx page or are you binding it dynamic ?
If you are creating it dynamic then you can do something like below.
ASPX
<asp:placeholder runat="server" id="plc" xmlns:asp="#unknown" />
Code Behind
Dim RegionList As New DropDownList()
RegionList.ID = "RegionList1"
RegionList.Items.Add("---Please select---")
For Each region As String In regionCountryData.Keys
RegionList.Items.Add(region)
Next
plc.Controls.Add(RegionList)