Hi Winton,
Thanks for your comments. I must admit I had little woodworking time since I built the vise and it has not seen enough use in a variety of situations to make conclusions about longevity. I can say this much:
- Dog block is built nearly the same way as Benchcrafted vise (same issue with thin side walls around dog hole) - mine is overall a little beefier. Considering that quite a few of those were built, we'd probably have heard if there were issues with that design, although it is very possible users of those are just gentleman woodworkers like myself.
- Kreg rails are pretty strong, and cranking the vise harder than I can imagine needing produces no discernible bending of rail walls. There are a couple of reasons for that: top rail wall is thin, but is supported by the bench so there is nowhere to bend, bottom wall is a lot stronger/thicker because it forms the T-rail and you can't really flex it. If using ordinary mitre channel, I would definitely screw in a strip of hard wood under it to prevent outward bending on bottom. Due to length of the rails, there is no stress near ends of the rails, where it would be most prone to bending.
- I don't know how it would work for spreading, but I don't really see that it would perform very differently than clamping.
I went through this design in my head over and over and had a lot of concerns about strength. When I built it, I realized it is much stronger than I expected it to be. Alas, without more hard use, I can't make a definitive statement, but I will update this thread if anything ever happens to it.