Vagrant is a tool that spins up VMs for you and destroys them. The only real requirement it has is that a "base box" has been created before hand.
At that point, you can install the VM using different provider hosts (kvm,virtualbox,etc...).
The goal of vagrant is to unify VM environments for developers with production env. This is very similar to what bigtop aims at providing. Vagrant adds host/guest shared directories, static ips, and allthe other goodies that one has to configure manually, into vm provisioning in a vendor neutral fashion: Essentially giving a declarative API to VM creation.
I would like to suggest that bigtop provides / maintains vagrant startup scripts that layer hadoop tools on top of a "base box" vm. This is slightly different than the current strategy which creates a full blown VM with hadoop on it. The vagrant approach provides a means for more developer customization of the vm artifacts being used without adding any real overhead (other than having vagrant installed and understanding the very simply vagrant recipe for creating a vm).
Probably in the begining this could be complimentary to the boxgrinder created VMs, and over time, maybe people would migrated to using the vagrant provisioned VMs as they become more popular and use of vagrant gets more common in the community.