Lutri seems tailor made for Vintage. Because of the restricted list, Xerox builds have been pushed towards being mostly singleton anyways, so the concessions we make to have him as a companion are relatively minor in exchange for essentially having an eighth card in hand at the start of the game. The biggest loss is the three extra copies of Force of Will; they're in the sideboard, but our maindeck counterspell suite definitely suffers. The good news is Lutri himself can potentially serve as a counterspell if a counter war comes up. The other major concession is the loss of Preordain as a four-of. Faithless Looting stands in.
Edit note: I originally had an extra copy of Lutri in the main, but he was cut when testing found the redundancy to be unnecessary. It was replaced with a Dark Confidant. Flipping up a Treasure Cruise feels bad, but it's a relatively minimal chance we hit anything too painful (only five cards out of sixty with cmc 5 or more).
MAY BANLIST UPDATE
Having fallen off earlier in the face of a Lurrus-dominated meta, Lutri decks seem well-positioned in the coming months. I don't think it could be argued that there are any other companions comparable in quality for the vintage format now. That means that the advantage that comes from a companion can now only be realistically gained through Lutri. The question is just what the best shell is going to look like. This is my pick, but if anyone has ideas or things that I might have missed, please let me know in the comments.
JUNE COMPANION RULES CHANGE UPDATE
So is Lutri still worth it? Maybe. Probably not. If Force of Will gets restricted for some reason then absolutely. Vintage games don't tend to have lots of opportunities to just pay {3} at sorcery speed to draw one card. I might experiment with putting him in a more controlling or ramping shell, so I might still update this list to make him work.
ON CARD ASSESSMENT
So why exactly is Lutri powerful enough to justify cutting extra copies of important cards? With Lutri in the sideboard, we have access to eight cards at the start of the game. Think of it this way: we have, from the start of the game, the ability to pay three mana at instant speed to double the effect of one of our powerful instants and sorceries. It's like they all have extra text that reads "Kicker {1}{U/R}{U/R} / If this spell was kicked, copy it and make a 3/2 Elemental Otter." Four mana body that draws six cards at instant speed? Sign me up. Not getting the cards you want? Pay five mana to go get any two cards out of your deck. Removal and draw spells can be turned into two-for-ones for the measly cost of three mana and NO cards in hand. Also Time Walk can become a Time Warp that copies itself immediately (or a half price two-piece Time Stretch, depending on perspective). While it is only usable once per game (barring the use of Ephemerate), this is vintage, so the games aren't going to last for very many turns anyways, especially if we can Lutri a Recall or Time Walk. In such a situation, the worst case scenario for us is we're two-for-oneing if they have multiple counterspells up (Flusterstorm, as well as one or two other relevant counterspells in the format, doesn't hit Lutri). We make sacrifices for it, and it's a little hefty on the curve for three, but its effects scale with the power of the spells in the deck and in Vintage we have the best spells ever printed.
As a final aside, the last crucial component of understanding the power of Lutri is realizing that he is an adorable Elemental Otter. Look at 'im. Look at 'is li'l face, chasin' spells all day. Imagine summoning this cheerful good boy to your battlefield and seeing him copy some ridiculous efficient spell. Worth it.
