Selecting a V/S higher than the a/c will climb in VNAV(managed climb??) is a very, very, bad idea.
The risk is that you'll select a V/S rate of climb greater than the a/c is capable of flying. Even with CLB PWR the aircraft will slow. Get too slow and you'll be unable to climb. Get slow enough and you can be forced to descend to regain speed or get so slow that the plane gets into slow speed buffet. It can get very ugly.