Well: haggis is sheep (typically, mostly) offal, oats, and spices… vaggis is pulses, nuts, and finely-diced winter vegetables, oats, and spices. It’s just what you’d expect.
And it doesn’t taste bad, either! It lacks the dry, meaty tang of a sheep’s pluck, but it’s a perfectly acceptable substitute. Several of the major haggis manufacturers now market a pre-made veggie alternative haggis, so a little shopping around should put you in possession of one. Just steam it just like you would the legitimate article. Alternatively, there are recipes online if you want to make your own: but unless you have sausage-making gear, it’ll necessarily be a baked haggis.