The VHF vertical won't affect the way the MQ-26 quad works, at all. You really should not need to use a fibreglas mast, either -- that quad works fine with an aluminum or steel mast, as it's horizontally polarized.
People do this *all* the time.
In reality, you could probably mount the VHF vertical right in the middle of the MQ-26 (literally, at the same height as the quad, stuck in the middle of the quad) and that wouldn't impact the way it works, either.
WB2WIK/6