Allow me to interject a little of my experience with both vac-packed and non-vac-packed curing.
Regular curing (non-vac variety) is sufficient and has been for a long time. However, the meat is cured simply by having the salt/cure diffuse into the meat at a rate of its choosing. In addition, the surface of the meat darkens, due to an oxidative process; this process in no way harms the meat and disappears with cooking usually.
Vacuum packaging does allow the cure to penetrate the meat more quickly and, when using nitrite based cures, allows the meat to cure more quickly. Because the process excludes air/oxygen from the package, the surface of the meat does not darken, retaining a more pleasing initial color.
I use vac-packing mostly for Corned Beef, where I consider a nice color to be a real plus, both from a cosmetic standpoint and from a technical standpoint. I leave my briskets in the vac bag for 1 days per pound and it has seemed to be entirely adequate so far. However, a 1.5 day per pound cure would do no harm, because with Corned Beef, one can change the cooking water if it is too salty. BTW, I usually use Morton's TQ, too.