Can you access any other resources apart from the Exchange server?
Do your work and home network subnets overlap?
1) Verify connectivity by pinging devices on the other side of the VPN (some are configured to ignore/block ICMP, but routers are less likely to).
Fail: May not be connecting correctly after all. Fiddle with VPN/router/client firewall settings.
Pass: 2) Check DNS resolution for the Exchange server (assuming your profile is pointed at a domain)
Fail: Find out what it is resolving to, and by what DNS server (use ping, nslookup, and Wireshark if necessary). The outcome determines corrective measures.
Workaround 1: Modify your profile to access the server's IP.
Workaround 2: Specify an entry in hosts file.
Pass: Don't know what the cause is.
"No battle plan survives contact with the enemy." - Field Marshal von Moltke