Whatever works, but I feel like some deities would strongly prefer meat. Oðin leads the Wild Hunt and feeds his table scraps to his pet wolves - do you really think he'd be happy with a plate of tofu?
Also, plastic is the ultimate disrespect imo. It's literally fossil fuels. Far more destructive of living things than horn, provided the animal whose horn you're using wasn't factory-farmed. It frustrates me greatly when vegans talk about how harmful meat animal products are to the earth and then advocate plastic and soy substitutes. (The soy industry is utterly ruthless and a major cause of deforestation in South America.)
Yes, many of the gods would probably prefer meat (as do most Asatruar). Your path may be impacted by the god/desses with whom you have the most contact. I do not believe followers of Freyr historically would have even considered a vegetarian diet and, historically speaking, animal sacrifice to Freyr was common. However, He has many aspects that directly tie Him to the health/fertility of the land, and to peace and plenty. It is reasonable to develop a strong UPG (unverifiable personal gnosis) regarding Frey's position on modern agriculture and factory farming.
As far as soy goes, though I didn't mention it and use soy very rarely, it is in my opinion that buying organic soy is a must else you support a very harmful industry. Incidentally conventional soy is fed to factory farmed animals so eating conventional meat supports the soy industry where organic soy fights against it.
Regarding plastic, I make efforts to keep plastic from my day to day lifestyle and avoid artificial textiles. I mostly wear linen, organic cotton, hemp, and raglan. I mostly use glass and metal. Complete removal of plastic from the modern lifestyle would be an epic feat and if you have accomplished this, please let me know how you do it!
That said, for those vegan Heathens who prefer to make head nods to our ancestral culture, the site I linked to is the only resource I have found and I certainly searched for something classier. Perhaps in the future artisans will make stoneware horns. In the meantime, I have no idea where each and every horn maker sources their horns and so I can only guess what the cattle must have lived through.
Hail to you, vaguely humanoid, for providing an interesting addition to the discussion.