The way that I see it, and I may be missing something as it wouldn't be the first time, there is no practical difference between using a barrel vise or a "Reaction Rod"/Magpul tool to install an AR barrel. When torquing the barrel nut both are going to cam the upper against the index pin as it is the only interface that keeps the upper from spinning on the barrel extension when you're tightening the barrel nut. I've yet to see a tool that will address this while also holding the barrel.
The difference that I do see in using a barrel vise is that it may not be all that convenient to do so. The "reaction rod" and the Magpul tool are much more convenient tools for this particular operation.
