VPN's wont really affect capping as thats set by a 'hard' data transfer limit regardless of what protocol you use. It can, however, limit your ISP from traffic shaping based on protocol so it can definately help when your provider decides to limit a particular protocol (p2p, newsgroups etc).
The main problem is that VPN is itself a protocol and is relatively easy to detect. That means ISP's will simply start to rate limit all VPN connections if they see that protocol as becoming a problem.
Generally, whilst a vpn may work to get around a limit presently I dont see that it has much of a future in that particular role so I'd tend to use it for what its actually meant for - privacy.
Of course, if everybody used vpn's then the ISP's would have absolutely no chance of protocol based control. The downside to that is that few vpn's will actually run at more than a few Mbit/sec and even fewer run at anything approaching 50Mbit/sec so the ISP's will still see a reduced data flow - they win either way !