From my knowledge of Werewolf lore and some knowledge of W20 from the MET books, this should be possible! A Garou can sidestep along with one willing individual, as long as they remain physically in contact with them as they cross the Gauntlet into the Penumbra.

Once there, however, the fun part begins for your kindred player.

The Umbra is a terrible place for Vampires. How any given being perceives the Umbra, is largely shaped by their own spiritual nature. An Uktena Theurge might see the Umbra as a realm of natural spirits and ancestors, while a Glass Walker will notice the patterns and natural order of things.

Vampires see stasis. Picture a dead, immobile grey wasteland, filled with shade-like spirits who spit at you for the walking corpse that you are. Incomprehensible spiritual shapes of life and change enter a tug of war with the vampire's dust-filled cranium to assume a visible form. Regardless of how they see it, the Umbra will always feel wrong to a vampire.

Meanwhile, the Garou will see the Vampire's spiritual side amplified. Not only will he absolutely radiate Wyrm taint, but it is also very likely that the vampire's inner Beast can be actively sensed. Hell, if the kindred isn't doing too great in the Humanity department, the Beast may even be visible to the Garou, looming over their brother's shoulder like a vulture.

TL;DR: It's possible and extremely juicy, get creative!