When using a dial-up connection to VPN into the internal network, the packet size in Windows should be set to "small."...
In my years of working with internet dialup connections, I have discovered that no such connection should ever use larger than a small (576k) packet size. While this is not much of an issue with wide-band connections, it can be a killer with dialup. I discovered this when working for an ISP who used Cisco equipment to connect dialup access. The ebb and flow of connection speeds would occasionally cause the packet size to increase to medium and the client PC would promptly "gag" on it.
In the properties of the Tcp/IP->Dialup Adapter | Advanced Tab, select packet size and set it to small. The default is Auto.
Note: This will affect ONLY the dial-up connections.