You might try XMP5744 powder for your Vetterli, that powder was made for that kind of use. Trailboss works really well for reduced loads in "modern" bottleneck rounds, but I don't know how well it might work, or at what pressure, in a big bore, slightly bottleneck one.
A lighter bullet may help with the recoil, but since you have to keep the same OAL, it might require trimming cases to a longer length. Then you could use less BP and some kind of filler.