I use Verizon's BroadbandAccess/NationalAccess.
You may find problems using a VPN client, bu those problems are due to restrictions of the client software, not the network. The network allows VPN connections but you'll want to check up and see if Cisco's VPN client has any restrictions when going over a WWAN link.
As far as the network goes, it works just fine for VOIP in most cases. In NationalAccess areas sometimes the voice is a bit tough to get a good call on, but in BroadbandAccess areas generally the call quality is good.
Technically, all BWA providers in the United States restrict access to email, web surfing and Intranet applications. Realistically, it is unlimited. If you are abusive (hooking up Wi-Fi routers and using 5+ GB/month) then Verizon might terminate you rather than let you mess up service for others. I don't fall into that category so I prefer having the restrictions because that way there's less of a chance that a VOIP call will have poor quality due to the network being overloaded.