Your options are:
1. Users connect to the VPN when at home.
They could then open the project is project professional. Note that this
will be slow.
2. Get the users to check out the project when they are at the office and
then select File > Save Offline . They can then open the project at home and
it will be checked out at the office. They can edit the project and save it
offline. When they get back to the office and connect to project server,
they can select File > Save Online.
3. If you have Terminal Services / Citrix then the users could log into
terminal services from home and run Project Professional through terminal
services.
