Are you planning to spray the vineyard yourself? If your planning to do your own spraying look for a sprayer first and make sure the tractor you get can handle it. Some sprayers eat up a lot of horsepower. As for mowing, you will want a tractor around 60" wide as when the vines grow your 9' row spacing looks awfully tight and 60" wide tractors do fine in that row spacing.
John Deere, New Holland, Case-IH, Claas, Landini, Massey Ferguson, Fendt and a few others make standard fruit and vineyard tractors that are narrow, have plenty of hp, and a low center of gravity. You can find them used for a decent price if you keep your eye open but at the minimum look for a pure utility tractor (ideally around 45hp or more) that you can set around 60" as it will handle the mowing and any future vineyard related chores you decide to take on yourself.