We actually only utilize Citrix ICA onsite, but here's what I what I recall about MSTSC.
As you've realized, this is no issue with RDP5.1 because you can use command line parameters or a connection file.
However, RDP5 only has 1 command line parameter, the name of a connection as defined in Client Connection Manager. Also adding to the mix, in RDP5, MSTSC doesnt install to the System32 folder, it defaults to the Program Files but the user can install where they see fit.
You can get the exe path by looking to the Uninstall script.
set oShell = createobject("wscript.shell"

strPath = oShell.RegRead("HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Terminal Server Client\UninstallString"

strPath = Replace(strPath,"setup\Setup.exe","mstsc.exe"
You can either use "user-emulation" aka Sendkeys.
oShell.Run chr(34) & strPath & chr(34), 1
oShell.Popup "Launching Terminal Services Client....",1,"Please Wait"
oShell.AppActivate "Terminal Services Client"
oShell.Sendkeys "ServerIPorName~"
Inject the Client Connection Manager connection config via the registry and use it on the command line.
strKey = "HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\MyServer\"
oShell.RegWrite strKey & "Auto Connect",1,"REG_DWORD"
oShell.RegWrite strKey & "MRU0","10.10.10.10","REG_SZ"
oShell.Run chr(34) & strPath & chr(34) & " MyServer"
Jon Hawkins