some providers do not accept to use of google DNS, due to some security reasons, so they always supply your DSL connect DNS Settings to use.
After connecting to VPN to your office, this will handover to your office network.
then you need to look at what IP address on the VPN you get when working, and when not working, and report back to the network administrator to investigate for you.
also worth Looking at route print (working vs not working situation that helps)