You can do this using the PortalId of the other portal.   (Please note that there may be syntax errors, as I have not tested this code, but it should work.)

Dim PortalId As Integer = 1
Dim p_Portal As New DotNetNuke.Entities.PortalInfo
Dim p_Tab As New DotNetNuke.Entities.TabInfo
Dim ctlTabs As New DotNetNuke.Entities.Tabs.TabController

p_Info = DotNetNuke.Entities.Portals.PortalController.GetPortal(PortalId)
p_Tab = ctlTab.GetTab(p_Info.HomeTabId, p_Info.PortalId, False)

Response.Redirect(p_Tab.FullUrl)

Sorry about that...  Here you go.  This will give you comma delimited listing of the portal aliases for a given portal id.

Dim arrPortal As New ArrayList = DotNetNuke.Entities.Portals.PortalAliasController.GetPortalAliasArrayByPortalID(<portal_id>)
Dim strAlias As String = String.Empty

If arrPortal.Length >  0 Then

    For intI As Integer = 0 To arrPortal.Length - 1
        strAlias = String.Concat(strAlias, arrPortal.ToString, ",")
    Next    

End If