Dave: I would be very, very careful about using this load data. I have the VV Reloading Guides #1 and 3 and neither lists a load using N350 with a 250 gn xtp in the 45 Colt. There is data for N350 and 180 gn bullets in the 45 Colt that starts with 13 gn and goes to 14.4 gns at 1235 and 1405'/sec with 11.6 - 15.5K psi
For a comparison with the 44 Mag, the Guide lists N350 and a 240 gn slug with loading data from 11.9 - 13.3, at 1169 and 1294'/sec and 26.8 - 36.5K psi. The narrower case and heavier bullet of the 44 mag may result in higher pressures and although I am sure the Ruger Vaquero is strong enough to handle higher pressure loads in the 45 Colt, I would check on that before I use that load... in fact,to be on the safe side, do not use N350 with the 250 gn xtp until you get clarification from VV that it is a acceptable load for them.
You can contact Kaltron Pettibone of VV, in Bensenville, Il at 708-350-1116 and try to obtain clarification from them/him about that load.
I suggest this strongly. I was once given a set of 7.62x25mm Tokarev dies by Lee that contained load data that looked waaaay beyond anything I had previously read and I ran it past Ron Reed for clarification. The clairification I got was to stay the hay away from that Lee load data, which I have. Call VV and get clarification from them. HTH. Mikey.