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There are many way to control and select outbound paths, so you should be able to fairly easily redirect the outbound VPN traffic to pipe B. (Often I suggest egress QoS to manage which flows get what bandwidth when there's congestion.)
Inbound control, especially from the Internet, is much more difficult to control. As the other poster noted, you can cap inbound bandwidth utilization of the VPN traffic, but doing this downstream of the congestion bottleneck (your pipe A) doesn't always work as well as we would like. If the traffic is not rate-adaptive, it will use whatever bandwidth it can on your pipe. If it is rate-adaptive, it should slow as you drop some of its packets, but it may exceed the policer rate (and use your pipe's bandwidth) before it gets to your policer.
