Steve,
I just banded my 500 lb bulls and gave them BarVac CDT at the same time as banding. It takes 3-4 weeks for the balls to shrivel up and drop off so you do (usually) get the 21 days for the toxoid to take effect. In my case, I also gave the CDT a month back so they sould be good to go. It's been three weeks on my 7 calves and so far so good.
The goal should be to band earlier, but I'm still learning. My neighbor banded about 10 calves and he never gives anything and they're all fine.
The Toxoid (Barvac, Covexin) is a vaccine that provides long-term immunity, but takes longer to be active. THe Antitoxin is short-term and doesn't provide long-term immunity, but provides immediate protection. This is probably better for banding as it takes two doses of Toxoid for full immunity, etc. At least that's my understanding.
Milkmaid is right, though, if you want to give something at the time of banding the Antitoxin is probably best. In my case, I gave the Toxoid twice, a few weeks prior to banding and then another at banding.