A corporate VPN probably wouldn't appear in the lists of "known VPNs" or have low IP reputation; also, outside perhaps of some subtle timing issues, there's not

A corporate VPN probably wouldn't appear in the lists of "known VPNs" or have low IP reputation; also, outside perhaps of some subtle timing issues, there's nothing to distinguish client->corpvpn->service traffic from regular old probably-NATted traffic from that network.