Root Activate on boost

Is it possible? Yes.
It is complex? Yep. I don't even know all of the details myself, though others might.

Also, you'd need to redo the changes every time a new ROM is flashed.

The means required to switch carriers on a CDMA phone reaches into legally gray areas. As such, it's something that takes a little looking to find the right info. I don't have the knowledge myself; but I'd suggest starting your search on XDA and Google.

the first thing you should know is that Sprint does not want you to do it, so to make it happen you would basically have to circumvent their controls which would be a violation of terms of service on Boost and according to many violates FCC regulations.

Most of the CDMA systems have a known list of the ESNs of the phones they've made and will allow to be activated on their system. So Sprint has its list of valid ESNs and Boost and VM have their own, and no ESN from one is generally welcome on another. Boost will not allow a phone with an ESN listed as being from VM to be activated on their accounts, even though there is no technical constraints as VM and Boost are both Sprint subsidiaries using same towers/frequencies/etc.

As for if it could be done, of course it can, not too hard. The "theoretical" process involves putting the ESN of a "donor" Boost phone onto the VM phone, replacing the "unwelcome" actual VM ESN of the phone (the one that's on the sticker) and then activating it on Boost. Then the VM phone activates/provisions normally on Boost since it now has an ESN that is welcome in the Boost system. The "donor" phone would then have to never ever be used since two phone communicating on the system with the same serial number would trigger fraud alerts.

These are the theoretical steps only, as the process of altering a phone's ESN is considered by most boards to be bordering on illegal activity in the USA and therefore not welcome for open to discussion in a USA-based public forum. Additionally you would be violating the terms of service of Boost by performing those steps. I will refrain from stirring up this type of discussion of the right/wrong of this (which always spin out of control), and assume that you do not intend on violating any laws applicable for your jurisdiction