MtGGoldfish is going to give you a great picture of the metagame, and the top tier decks, but you shouldn't limit yourself to just playing those decks if you have a preference for another deck. Vintage is a smaller field than other formats and that means there are always good strategies out there that people just haven't figured out yet. I think you should feel free to build your own list from the ground up (it's good practice!) but there are some thieves lists from around April on mtgtop8. (I always use mtggoldfish for a first pass, but mtgtop8 usually has decklists from paper vintage tournaments that aren't posted on mtggoldfish). I might personally start with Ruben's list, just because I know he's played a lot of decks in the past that are my style.
Those lists all showed up before War of the Spark shook up the format, so I would consider the following things:
Tinker -> Blightsteel Colossus itself seems like it isn't any worse, Swords to Plowshares and Outcome decks have a smaller market share than they had before. Dredge and Survival have a bigger market share and Tinker is one of the better game one options there.
But Karn, the Great Creator and Collector Ouphe mean that Null Rod-style effects are everywhere. That means the artifact mana you use to support Tinker is a little worse. I don't think that's a death sentence, but I might not want to go all-in with things like Mana Vault and Lotus Petal. Alternatively you can keep the accelerants as long as you have plenty of removal or you're feeling lucky about being on the play.
Narset, Parter of Veils has cast her shadow over the Blue matchup. Expect every Blue deck to have some. That means cards like Preordain are worse, cards like Jace, Vryn's Prodigy are MUCH worse. Cards like Gifts Ungiven and Sleight of Hand are better, but it's debatable whether they're correct to run. Good news for you, however, Narset, Parter of Veils fits nicely in with the Thieves strategy, and has a Notion Thief-like effect when combined with Dack Fayden, or even more specialist enablers like Geier Reach Sanitarium
Force of Vigor is great, so don't rely too heavily on a key Artifact or Enchantment. Grixis Thieves don't tend to do this so you're probably fine.
I think, and this is more fuzzy, the format right now is more about interacting on the board rather than on the stack. You need to be able to impact the board early and often. Karn and Survival decks are fast and mostly play their hands out. Blue decks are mostly generating advantage through permanents rather than draw spells. You either need to be able to play that game as well as them, or you need to generate an overwhelming advantage before they get repeat use out of those cards.
You could probably also start with a U/R Dreadhorde Arcanist deck and add the Grixis cards you like, rather than starting with an older Grixis deck at all. That might be the surest approach of all.
That said, I think perhaps M20 will bring us another shift in the format, and any or all of this could change very soon. Vintage is moving very very quickly right now, so I wouldn't wait for someone else to give you permission to beat people up with your favorite deck. Blink and you might miss it