It is not possible an unencrypted RDP connection via a VPN tunnel to build.
Linux RDP Client, RDP Server MS XP Pro SP3
id="2101" severity="warn" sys="SecureNet" sub="ips" name="Intrusion protection alert" action="drop" reason="MISC MS Terminal Server no encryption session initiation attempt" group="110" srcip="192.168.100.11" dstip="192.168.200.2" proto="6" srcport="55196" dstport="3389" sid="2418" class="Attempted Denial of Service" priority="2" generator="1" msgid="0"
MfG E. Riedel