... I contacted Ruger, and they won't sell the vented slide separately. They also gave me their standard answer saying they don't recommend modifying their firearms. Can I swap the slides like mentioned above despite what Ruger says?
I understand why Ruger states that "they don't recommend modifying their firearms" - potential product liability issues are a major concern for many American manufacturers. That being said (& FWIW), the P-series pistols are apparently highly modular; I've seen a P91 that also functioned with a slightly modified P89 top half (9mm & .30 Luger), as well as a slightly modified P90 top half (.45 ACP w/ OAL kept to 1.135", to feed through a P91 mag). In addition, I've seen modified P95 (safety model) slides used on P85, P91, and P93 frames.
All of which suggests that Ruger tends to use very similar (or identical) dimensions for certain parts on different models; whether swapping parts between models is safe or not, however, might require evaluation by a professional gunsmith.
As always, IMHO, FWIW, YMMV, etc., etc.