Hi,
I can see that Apple (or any large Internet service) might prevent logins from two IPs (your Home IP and the place the VPN goes to).
However I would have thought you are logging out fully at the VPN location (presuming this is also related to your place of work) then the Home Login would re-establish this as the "first" login and the VPN one would be rejected.
I don't use VPNs and don't know if the Firewall on the Mac is used with them.
This might leave the situation where the Firewall is blocking the Messages app locally but the VPN is allowed.
You imply the iCloud ID is separate from the WORK Jabber ID.
Can you confirm the Apple ID that is used for the iCloud ID is unrelated ?
8:35 p.m. Monday; August 17, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad