It is possible that the VPN software you are using may result in the traffic not being recognised as VPN - that would account for slow speeds. You will probably need to do a Wireshark capture (or use the VPN whilst PN monitor from their end) so that a check can be made that it is being given the correct priority. If it isn't they will add the appropriate signature which should then sort out the problem.
jelv
Plusnet ADSL PAYG Jan 2004 -
Plusnet Dialup Nov 2001 to Jan 2004
Previously Compuserve, BT & LineOne Dialup